Time Machine couldn't complete the backup to external drive
I keep receiving an error (Time Machine couldn't complete the backup to "Backup Plus". An error occurred while copying files.) when trying to backup with my external drive. It has been working fine the last year and now it's not. I have run disk utility and did first aid on the device and it is fine. It runs through but then at the end I receive the failure message. Any ideas? Using a MacBook Pro and running on Catalina the latest software
MacBook Pro 15", macOS 10.12