Hey there! Good question.
There maybe several causes, one question, is this the first Time Machine backup you’re attempting on this external drive?
I’d suggest following if you haven’t come across it yet:
If you can’t backup or restore your Mac using Time Machine:
https://support.apple.com/en-us/HT204157
Another good resource:
Time Machine Troubleshooting on Mac:
https://support.apple.com/guide/mac-help/time-machine-troubleshooting-mh15653/mac
Otherwise, can you provide more specifics? Such as if this is a new external drive, if you have backed up to this drive previously, and if so, if any changes were made before this started to happen? And also, any steps done to try to fix it so far?
Happy to help if I’m able to, as any of the people here, thanks.