If you cannot find the "Blocking" menu, use the search bar inside Settings. Type "Block" and it will appear instantly.
Facebook has a built-in cooldown period to prevent harassment. If you block and unblock someone rapidly to spam them, the algorithm stops you. You usually need to wait 48 hours after unblocking before you can send a new friend request. unblock user on facebook
Before we get into the technical "how-to," let's address the elephant in the room. Why did you block them in the first place? If you cannot find the "Blocking" menu, use