I understand you're having trouble deleting annotations on your file using the backspace key.
I just tested it on my end, and I was able to delete a measurement display using backspace. Could you please try these quick fixes to see if it resolves the issue?
Close any Figma tabs you're not using.
Browser: Force-quit your browser and open Figma in a new window.
Desktop App: Force-quit the Figma Desktop app and restart it.
If the issue still remains, to help us prepare to connect you with the appropriate team, would you be able to check a few things for us?
Does this issue occur in the desktop app, browser app, or both?
Is this issue occurring in a specific file or in all files?