Because you did a clean install on new motherboard, Windows can’t “pull” the old activation automatically unless you either have a digital license linked to your Microsoft account or enter a valid 25-character product key.
There isn’t a separate “Windows 11 code” you need to request, Windows 11 activates with the same type of license (digital license or product key). Go to Settings > System > Activation and try Troubleshoot, then choose “I changed hardware on this device recently” and sign in with the Microsoft account that was used on the old Windows 10 install.
If you have a Windows 10 product key, you can also try entering it directly in Windows 11: Settings > System > Activation > Change product key (make sure the edition matches, Home key for Home, Pro key for Pro).