I built a house for a NPC but none are coming. What do I do?
this Is the house I built for the npc
Two possible problems:
1) The very centre of the room might be too dark - try placing a torch somewhere there.
2) (this one's far less likely) There's a maximum size to a room - that room might be a little on the large side for an NPC.
In any case, as Robbie mentions, you should have a Housing Query tool that will tell you if a room is suitable for someone to move in - if it doesn't, it'll tell you what the problem is.
There are a few reasons I can think of that would cause NPC's not to move in.
You may not meet the requirements for anymore NPC's to move in. The guide moves in to any available housing so if he isn't moving in then there is something wrong with the house. The easiest NPC to get is the Merchant which moves in once you have more than 50 silver coins.
Your house may be to close to the corruption or crimson. You only have to worry about this if the corruption is right by your house.
Your house must take up less than 750 tiles of space. It seems like your fine here but you may want to decrease the size a little bit. Houses can take up as little as 60 tiles and still be moved into.
As QuadrAlien mentioned the center may be too dark. This also doesn't seem to be the problem but adding a torch in the center could help.
The easiest way to figure out what the problem is would be to go into your inventory, navigate to the housing menu, and use the housing query. It will tell you if your house is valid and what to fix if it's not.