When a post has a pending suggested edit (an edit by an anonymous user or one without edit privileges), it needs to be approved before further edits can be made to the post.
This includes retags, as they are classed as edits to the post.
I think I encountered this problem previously as well: If you approve an edit, but it needs more approvals before the edit review is completed you cannot make any further changes. If you see a mistake just after you approved the edit, you are out of luck. ;)
Apparently there is no mid-ground between, choosing to improve the edit immediately, and waiting for ...