Before changing any iCloud settings, I’d first check whether the files are still in iCloud Drive.
On your Mac, open Finder > iCloud Drive and look inside Desktop and Documents. Also sign in to iCloud.com, open iCloud Drive, and check both Browse and Recently Deleted. Deleted iCloud Drive files can normally be recovered for up to 30 days. → Recover deleted files on iCloud.com - Apple Support
For now, I wouldn’t turn off iCloud Drive, disable Desktop & Documents, or sign out of your Apple Account while you’re trying to locate the files.
If the files aren’t visible in iCloud Drive or Recently Deleted and the Restore Files page keeps giving that error, I’d contact Apple Support so they can check the iCloud side of this with you.