@TRACICRISPEN ...

I just tested adding an existing rubic to an assignment that I have in my own sandbox course, and I was able to add it without any issues.  Have you tried clearing your browser cache/history, closing your browser, and then restarting your browser?

• How do I clear my browser cache on a Mac?
• How do I clear my browser cache on a PC?

Or, have you tried using a different browser such as Mozilla Firefox or Google Chrome?  I've also had good success with Microsoft Edge on my Windows 11 laptop.

• What are the browser and computer requirements for Instructure products?

If none of this works, you might want to reach out to Canvas Support to see if they have any suggestions for you.

How do I get help with Canvas as an instructor?

Good luck, and keep us posted!