I found this thread and post with Google. This is the closest problem to mine but not exact.
The Google AI answers to this were Draconian. Delete and re-install your game. Excuse me. That should be the last option not the first.
In my case I have no mods or CC and never have had. The only thing installed in my game is any packs purchased with the EA App.
I observed the Enclosure option vanished from the Treehouse immediately after upgrading to the sliding pole. What I did is delete the treehouse and sell it off from the family inventory. I also sold off the slide and pole. Apparently, they are separate objects. I then bought a new treehouse and had my sim build it anew starting from scratch. The new treehouse has the enclosure option again but with no upgrades because the aren't built yet.
I suspect the pole upgrade broke the treehouse object as if somehow its presence obstructs entrance to the enclosure. I am going to avoid that upgrade and see if the bug comes back. I think it is important that those who use no Mods or CC post about bugs they experience to remove that variable from the mix.