@ MartinZ
I can't give a screen shot since I had uninstalled and did a fresh install, but it happened with v. 2021 (and not previous versions). I happens in the GUI when you click to Register, and that I believe is where the glitch is. It also will not let you cut and paste your license there either. After spending a LOT of time trying to get it to work, I finally did an uninstall in Safe Mode and fresh install, rebooted, then cleaned the machine, then tried to register and it finally worked...hours later.
I have also read this same thing happening to other users with various OS's, with the same version of Avast Free. So this may be should be something to look into. Thank you.