Time Machine problem

I am running Big Sur 11.7.4. I back up periodically with an external drive using Time Machine. It has always worked fine until my last successful back up in January 2023. Now when I try to back up I get this message: Time Machine couldn’t complete the backup to “G-DRIVE mobile USB”Time Machine did not finish backing up because some files were unavailable. Backups will resume when your Mac is unlocked. As far as I know, I haven't changed anything. How can I fix this so I can back up?