Hi ALC_7141,
We're happy to provide some assistance.
Based on your original post, we recommend the troubleshooting steps via the link below since you cannot save files to iCloud.
Add your Desktop and Documents files to iCloud Drive
To perform the troubleshooting actions recommended by the article above, please read through and complete the steps in the section titled "Store your files in iCloud and save space on your device."
We're recommending the steps above because they directly address the issue you initially reached out to the community about, which is that you cannot re-open files that you were previously working on because you can't save them to iCloud.
If the original problem persists after trying our suggested troubleshooting steps, then give the steps here a try: Access and manage your files in iCloud Drive.
Please provide us with specifics on what you see for more help if your issue has not been resolved, and we will continue from there.
Thanks for using the Apple Support Communities.
Cheers!