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.