Can’t Add Corner Room

[Miretar]

I cleared the wall but still can’t add the room:

 

[HaleyRocks]

You have to click upon the green colored space, so it becomes red. Then it is built!

 

[FilthyGorilla]

Sometimes the game has a bit of an issue registering your clicking when constructing the expansions I've found, just click on it rapidly a few times and it works, there's no harm or risk in just spamming it so you're all good