This error could be related to APFS.
Time Machine will produce lots snapshot files during automatic backup.
Need to use Terminal to delete the files one by one.
Search command:tmutil listlocalsnapshots /
This command will show all snapshots files, as below.
com.apple.TimeMachine.2018-04-30-021643
Delete command:tmutil deletelocalsnapshots snapshot_filename
Here, the filename needs to type like 2018-04-30-021643.