Some items in the Trash cannot be deleted because of System Integrity Protection
sudo rm -rf /Volumes/ExternalDrive/.Trashes/
Replace "ExternalDrive" with the actual name of your external drive. Be careful using the rm command and make sure the path to the External Drive is correct.
I ran into this trying to delete old backups. I resolved this by going into Trash, right-click Backups.db, Put back. Then following: How can I manually delete old backups to free space for Time Machine? (sudo tmutil delete /Volumes/<drive>/Backups.backupdb/<backup>)