Win 10 won't validate Win 7 product key

The free upgrade is over (ended July 2016 I believe if my memory serves me), you would need to purchase a new Windows 10 Key.

I don't think so.....I have upgraded about 5 computers over the past week or so...using the same method...

Some OEM Windows 7 licenses are not upgradable to win10.

When prompted for the windows license, skip it for now to get the install finished and then enter the key later. You might have to call licensing support to register the key that way.

You may wish to do some serious research on this subject, because the offer for Free upgrade did indeed ended July 29, 2016.  See: https://support.microsoft.com/en-ca/help/12435/windows-10-upgrade-faq  The fact that the process works, in no way means it is legal.   There are numerous discussions on this online.  Sadly, MS has remained silent on the matter, but their web page is pretty clear on the matter.  I certainly would not use this method for commercial/business PCs.

Now that Microsoft has made Windows 7 end-of-life, and Windows 8 was a colossal flop, they no longer have reason to offer free upgrades.  As Daniel says, the free upgrade program ended some time ago.

Some systems have a built-in BIOS key that allows them to run different versions of Microsoft operating systems and my thinking is that it is the BIOS key which permits switching to Windows 10, not the COA on the case.  BIOS key permissions can vary even in the same models depending on when it was purchased and who it was sold to.  A Brand X model Y system sold at retail could very reasonably not have identical BIOS key OS permissions as an identical system sold into the educational or government market.

I have upgraded about 5 computers over the past week or so...using the same method.

I've spoken to Microsoft support regarding this issue.

Some Systems will upgrade to or install Windows 10 using a Windows 7 license.
This is because (during the free upgrade promotion) a upgrade was reserved for that PC

So yes, you've done this successfully recently with a few.

This one however will need a upgrade license

The fact that the process works, in no way means it is legal.

Actually according to Microsoft support (referencing my earlier comment for context) generally speaking as far as they are concerned, if a legitimate windows 7 key or install is accepted "and" a digital license is successfully obtained then the install is considered legitimate

I've never seen any key that does not work. It still works and MS tolerates it, else they would have shut it down. You have no legal base, but I don't think anyone at MS will ever come after you as an individual. If you are interested, see this https://borncity.com/win/2019/11/30/why-the-free-upgrade-to-windows-10-still-works/ (still: no official statement, just some (wannabe-?) insider chatting).

Please note that nothing needed to be reserved in time. Also note that win10 1507/10240 setup will NOT accept the win7 key. You would need at least version 1511 of setup.

I'll just throw this link out to you as well. It is update on a somewhat regular basis so the information should still be valid.
https://www.zdnet.com/article/this-free-windows-10-upgrade-offer-still-works-heres-why-and-how-to-get-it/