Hi GILLIE1952,
Thanks for reaching out in Apple Support Communities. I understand that you're seeing an error message when you try to open files you've stored in iCloud Drive. I also see you've tried signing out and back in to iCloud, but the error persists. I can see why you'd reach out about this; I'd like to help. First, try signing in to iCloud.com with your Apple ID; are you able to access your documents there without an error?
Do you have another macOS or iOS device you can use to check accessing this data using iCloud Drive?
I'd also suggest checking for a configuration profile on your device, the message you're seeing can be associated with a managed device: Install or remove configuration profiles on iPad
If this is a device that's managed by a company, I'd suggest reaching out to the IT department before making changes to any installed profiles.
Let me know what you find.
Best Regards.