*- Console can be opened with ç, ~, ´, ö, or \. It differs between keyboard layouts.

You won't be able to place anything if there's an npc blocking the way. They will just come back if you command them to leave. You can also open console, click on them and then type disable to delete them.

If you're getting issues snapping in the door to the doorway just turn collision on again, exit building mode, then go back to building mode and it will work.

PS: This also works with everything else. When you grab an item while your collision is turned off, that item no longer collides with anything else until you turn collision back on again, which means you can build anything in it or around it without that item getting in the way. That toilet for instance, I put a regular chair inside it using this method.

Additionally, the textures in the doorway's location flicker between the doorway's textures and the floor's textures since they're both in the same place. Credit goes to [K.lo] el tigre for the easy fix, just open the console, click on the doorway and type setpos z -.01 to drop the doorway just enough for the textures to stop flickering. The doors won't get stuck if you just drop it 0.01