Deleted Time Machine Backups in the trash cannot be emptied because of System Integrity Protection (SIP), introduced in Mac OS X 10.11 El Capitan. If a Time Machine backup fails or gets interrupted then incomplete backups can result. The Time Machine will be unable to successfully complete a backup until this corrupted file is deleted. However, if you try to empty the trash, such backup files get stuck on ‘Preparing to empty the Trash…’. Deactivating System Integrity Protection enables emptying the trash in macOS 10.13 High Sierra, macOS 10.12 Sierra and Mac OS X 10.11 El Capitan.
