Hello BaffledOldLady,
If your iCloud is incorrectly showing that the storage is full, first try saving something directly on the iCloud website to see if you get the same error there. This will help isolate if the problem is coming directly from iCloud, or is the issue is coming from the computer.
If you can save files without error on the iCloud website, then try signing out of and back into iCloud on the computer.
Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac
Also make sure you've covered the basics like restarting the computer and making sure it's up to date.
Shut down or restart your Mac
Update macOS on Mac
If you're still getting the error that there iCloud storage is full, you'll want to contact Apple to report the issue: Get Support
Cheers!