How do I place a door?

I'm trying to place a door, and nowhere seems to be a valid place. Do I need something else before to place it against? I've built a small basic square 4 walled structure, with a gap left a little wider than the door.

Solution 1:

Doors will only fit within specified structures, you don't put them between two other structures. For example:

Other examples of the structures you can fit doors to include these two, plus the various entryways in the prefabs:

Solution 2:

Doors can only be placed in walls that have an opening for a door, you can't leave a gap and place a door in there.

When you are placing walls, scroll over to the right and you will see some that leave an opening for a door, use those walls if you want to place a door there later.