1
Open the Settings App.
Go to System - Activation.
Please provide a screenshot of that window.
2
In the meantime, open File Explorer.
Paste this into the Address bar and press Enter:
%AppData%\Microsoft\Windows\Themes
In the resulting folder, delete the TranscodedWallpaper file and also the Cached Files folder
Restart (not shut down) tour PC to apply the change.
Then try setting your preferred background wallpaper.