Hello, JimmyCrackCorns.
Thank you for using Apple Support Communities! I understand that after updating to macOS Catalina, you're unable to access your photo library in the Photos app or iCloud due to an error message.
If you haven't already, try using the Photos library repair tool on your Mac:
How to use the Photos library repair tool - Apple Support
If you have any available updates for Catalina or the Photos app, I recommend installing those as well: How to update the software on your Mac - Apple Support
You can find additional tips, including how to back up your library, here: Get help with the Photos app on your Mac - Apple Support
Let us know if this helps! Take care.