
How do I get the room on the left to be a valid house for an NPC?

How do I get the room on the left to be a valid house for an NPC?
There are two things which are causing your housing to become invalid.

Firstly, you can't use a platform as your only flooring - NPC's will want some kind of solid ground to stand on.
Secondly, because the platform isn't a valid floor, it's counting the circled area as part of your house. The whole area behind the house needs to have a back wall - this cannot be a dirt wall either, it needs to be made of wood or stone or somesuch.