This. You could also try using the activation troubleshooter's "I changed hardware on this device recently" option, as the upgrade can sometimes trigger that. Another option is to manually enter your Windows 10 product key in the activation settings.
+1 I had the same problem. What finally worked for me was a clean install using the Windows 11 Media Creation Tool, then activating with my old Windows 10 key.
Agreed. You could also try running the Activation troubleshooter again, but choose the "I changed hardware on this device recently" option this time. Good luck!