Pretty simple one, but caught me out for a few moments! This cropped up whilst tidying up and removing a redundant Organisational Unit (OU) within a domain I was working on.

Active Directory Domain Services
You do not have sufficient privileges to delete {OU Name}

I am an Enterprise Admin (EA) so knew it wasn’t permissions related. Someone in their wisdom had ticked the ‘protect object from accidental deletion’ box.

Untick this box and you should now be able to remove the OU.