how do you put a door on a wall?

i feel stupid asking this but im new to the editer and ive placed down a town with shop signs but every time i want to place a door on a wall it says *this must be placed on a wall* can someone please help me i just need to place doors for my stores and main quest giver and the map would be ready to hit steam

SHIFT + click to select the wall part you want to place the door on.

I recommend reading the tutorial. It answers this (and other) questions.

ok thanks i have dylixca so when i read the manual the first time my mind probly jumped over it lol XD
ill read it really slow this time

I also discovered that pressing TAB will cycle you through the 4 walls of a selected square! This was a godsend for me, as i use a VERY high resolution tablet and the UI elements are tiny.

thanks i didnt know that i also run on a tablet computer