Cannot delete Time Machine backup off external hard drive

Why do you want to delete the Time Machine Backup Drive?

You can not and should not manually delete backups from Time Machine. Time Machine will automatically thin and delete old backup when necessary. If your Time Machine Drive is full and automatically deleting old backups, then you need to use a larger external drive for Time Machine.

If you want to use the drive for something else? Then you need to use Disk Utility to Erase the external drive.

IF you are attempting to do this via Finder that is a major no no and a good way to totally mess up Time Machine Please describe how you are attempting to remove a backup off of TM. BTW, in most cases this action is totally unnecessary!