AT&T speed test cancelled due to browser tab switch. What does that mean?

Try https://SpeedTest.net as an alternative.

And... If you're running an Ad Blocker, be sure to turn it off for the ATT Speed Test URL + see if that fixes the problem.

Did you change over to another tab? Guess they want you to keep focus on the tab where the test is running

I did not change over to another Tab.  I was just watching the speed test with no other applications running, nor any other tabs open.

I am not running an ad blocker (unless Norton Anti-Virus) is doing so.

My laptop has a wireless adapter and a RJ-45 adapter.  
I disabled the wireless adapter, and connected a CAT 5e cable to my computer.
The speed increased dramatically.

When I enabled my wireless adapter, the computer ran very slow.

To check which apapter was being used on my computer for interent, I googled for a solution.  It said to check the priority of the network adapters:

1. Click Start, then click Run
2. Type ncpa.cpl
3. Click Advanced
4. Click Advanced Settings
5.  Under "Connections", my wirelss network connection appeared first, then my local area connection.
6. I clicked on my local area connection, then I clicked on the UP arrow.
7.  Now my location area connection has first priority, and second was my wireless connection.
8.  I shutdown my computer AND I unplugged the power cable to my cable modem.
9.  I plugged in the power cable to my modem, waited a few minutes, then powered on my laptop.
10.  Now my download speed is 146 Mbps, and my upload speed is 24 Mbps.
11. My computer is running very fast on the local area connection!

So far, when I run the ATT speed test, I am no longer getting this message:  "Error:  Test Cancelled due to browser tab switch"