Have you rebooted and tried to delete?
If so and it didn't work try booting into Safe Mode by booting with the Shift key held down and try deleting from there. Then reboot normally.
NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following;
• Verifies your startup disk and attempts to repair directory issues, if needed
• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)
• Prevents Startup Items and Login Items from opening automatically
• Disables user-installed fonts
• Deletes font caches, kernel cache, and other system cache files