See if killing your explorer process fixes this:

Cut and paste the following into Notepad

@echo off
taskkill /F /IM explorer.exe
explorer.exe
exit

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Save to your desktop as wm.bat
then right-click on it and choose Run as administrator

You should get a "success" message and the watermark disappear

If so then restart Windows to check it has gone for good!

It's pretty simple.  Activate Windows.  This has nothing to do with Windows 10.  You have not entered a valid product key and activated Windows.  Do so.  It's telling you what to do - Go to Settings!

Lee is 100% correct. Activating windows is proving you own it. It isn't about Win10.

Indeed open and administrative command window,
execute the following command

slmgr -ato

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If it does not activate correctly then you need to enter a valid code, (unless you are on a network that has a corporate KMS Server) If that is the case speak to your IT.
If its you own machine then you either need to enter a retail product code or a MAK Key if you own one. that's a 25 character long key you enter it with the following command

slmgr -ipk 12345-ABCDE-12345-ABCDE-12345

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If you dont know what version you installed then run the following command and it will tell you

slmgr -dli

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Once it accept the correct code then simply rerun

slmgr -ato

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</P>

MASQ: we did get success but the watermark remained after reboot.
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Before continuing with the other suggestions why would we need to activate Windows on a refurbished system we bought from Amazon?
I think the key is on the bottom of the laptop and I think we have Windows 8 or we were sold this system as Windows 8. I found system tools and it says the operating system is Windows 10. I don't think this watermark was there when we bought it. Is this prompt to activate the free upgrade that was given to many Windows 7 and 8 users?
The other day I may have clicked activate and got a red error message.

If it was updated from 8 to 10, then it should have an electronic key.  Right click This PC select properties then select activate windows in the area where it shows the windows activation status.  Follow the instructions to get it activated.

From Fred's more simple path:

Yep just click on activate windows to get activated (if it has an electronic key from the windows 8 update process).  These are usually stored at Microsoft.  If you have any issues poke around the help to find a phone number for activation.  Microsoft has folks to help with activation.

MASQ: Did you get any other messages over the past few weeks warning you that the system was going to become inactivated?
.............no and before this trip the laptop had not been used for more than 120 days.

The key on the bottom for Windows 10 pro is the standard 25 character code or serial number.
-------------------

I went to change product key and entered the key on the sticker or the bottom of the laptop and no success.

What did slmgr -dli reveal?

Run the command prompt as administrator!

Done

Was this after adding the Key in my post or was that key rejected?

It's showing that key as the one installed.

You mentioned a key sticker on the base of the unit. What's the last block of characters in that?

Is there a make/model? Afraid this is increasingly looking like it's not activated because the key used is invalid.

Was this after adding the Key in my post or was that key rejected?
...............the key on the sticker is seen in the 3rd photo above.
It's showing that key as the one installed.

You mentioned a key sticker on the base of the unit. What's the last block of characters in that?
................see 3rd image above
Is there a make/model? Afraid this is increasingly looking like it's not activated because the key used is invalid.
.................Lenovo T430S

Here is the bad (?) news.  The only key I have is shown above in one of the command windows.

MASQ
I bought this as a Windows 8 system. If it came with Windows 10 free upgrade, I do not know.
-----------------------------------------------------------------------------------
My friend bought a Windows 7 from Amazon and he has a disc that came with his desktop computer that gives him a free Windows 10 upgrade, just install the disc and follow the directions. It's good forever I guess since he has not yet used the disc and his 7 system is 2+ years old.

My laptop desktop looks like my home desktop computer which does have Windows 10. Local shop upgraded it back in May when they did a repair. It was a Windows 7 desktop and they said the hardware could easily support Windows 10 so they gave to me at no extra cost.
I said sure go ahead since early 2020 I would need it any way.
----------------------------------------------------------------------------------------------------------------------------------
I got no disc with the laptop and from the sticker:
Windows 10 Pro for refurbished PCS
BTG (the B might be an 8)
Then the key as shown in an image above.

A bar code and under the bar code is:
03307-126-556-227    X20-88770

COA for use with refurbished PC only - no commercial use.
---------------------------------------------------------------------------------------------------------------------------
The Neosmart link led me to a forum. Here is a comment by someone with the same issue.
>>> They responded early yesterday and some of what they say you also know.
Title:
Windows 10 Lenovo laptop and inability to activate.

Comment:
Embedded keys replace the sticker in later PCs.
If you have a sticker the second screen is inevitable as there's nothing to find in the firmware.
Is the sticker for W10 or a previous OS which has been upgraded ?
My W10 was a free upgrade from W8.1 and never required a key so I'm not entirely sure of the consequences on an un-activated system but I believe the only consequences are the nag screens and unavailability of some system commands but others may correct me on that.
------------------------------------------------------------------------------------------------
So if I have Windows 10 Pro and it works fine which it does though I am in a foreign country and I do not know the internet speeds, and I only use this laptop for travel which is at most (3) 5 week periods in a year, I would not want to have to remove O/S because I don't know how and the cost is not worth it.
As the person commented my question also asked about the consequences of doing nothing and just pretend the "activate" watermark is not there.
Any other negative consequences other than what they say.
-------------------
In this comment I have not addressed everything in your previous comment, I'll wait to hear back from you after the new info I am posting now.

Thanks.

I just picked up a refurbished desktop.  It came with Windows 10 and it booted into the Windows initial setup.   After getting it setup, I had the same message, Windows not activated.  I called the vendor, he told me to go to the Windows activation screens and enter the product code from the sticker on the case.  The sticker was a Windows 7 sticker, I had my doubts, but I tried it.  Voila it worked.  Activated Windows and all is fine.  It must be some sort of activation method for refurbishing.

I tried the key on the sticker on the bottom of the laptop and it failed.

If I just stay un-activated what might be the consequences.
No critical updates, or maybe Microsoft voids my key for failure to activate.

Sorry, I misspoke in my earlier post.  The Sticker was for Windows 7.  I edited the comment so as not to mislead anyone.  Again sorry.  I should never write from phone, I get too distracted.

My sticker is a Windows 10 Pro product key and should work. It was sold as Windows 8.1.

Sorry for the absence

From your sticker:
BTG = Blair Technology Group one of Microsoft's authorized refurbishers so the Key should be good

You don't have an embedded key so it needs to be added at a prompt, the Key is labelled as good for a refurbished machine with Windows 10 but in your 1st post you say it came with Win 8 so I'm still trying to get my head around the fact it's got a Win 10 Key sticker and is now running 10.

How did the upgrade to Win 10 Pro happen?  Was this at the local shop or are you talking about your Desktop at that point?

I'm struggling to see how a refurb certificate and key for Win 10pro is attached to this if you didn't get it with Win 10 initially.  Normally people don't hand over their PC to a local shop and get it back as a refurb with a different operating system(!)

In answer to your "what happens if I ignore the nag?"  You'll get some limitations in function and then some full screen nags happening but on the whole as a basic user you're not going to see much of an issue other than the current cosmetic one.

After travel I'll go to my Amazon account and see what I bought, Maybe it came with Windows 10 but it's been so long that I'm not sure I could have gotten a Lenovo brand, refurbished for the cost I paid which I think was under $225. Maybe it was $323 and not $223 but I know I got it free with Amazon gift cards.
Even if it came with 10 and thus the sticker then not sure why it was not activated by the seller, or a disc or other document came with it.
I'll know and close the question by the end of the week.

I was able to get this from the subject line of an Amazon email dated June 2018.
We thought you might be interested in knowing that customers who bought "Lenovo Thinkpad T430 Built Business Laptop Computer (Intel Dual Core i5 Up to 3.3 Ghz Processor, 8GB Memory, 320GB HDD, Webcam, DVD, Windows 10 Professional) (Certified Refurbished)" also bought these items............

It was a Windows 10 system (my error) and the sticker is on the bottom but it won't activate.

best contact MS then, and ask for their help

thanks