Hello.

I recently download a mod for nightlife ep and it has changed the in-game build functions. Now I can not place doors in walls, or anywhere for that matter. What can I do to fix this problem?

remove the mod that might help
also exactly what do you mean by mod?

Try typing "boolProp snapObjectsToGrid true" into the command window.

This is the wrong place for this thread,in the first place,and try moving the "mod" from your downloads folder,then see if it works.If it does and you still want the mod then move it back to your downloads folder.Then test again.

in addition to jaxads suggestion, if it still doesn't work, make sure moveobjects is off. even if i accidentally leave this set to on i find i can't place doors or windows. before NL it was ok, since NL i find it isn't
out of curiosity, what mod did you download which has caused this effect, if it is that? no name rank or number necessarily, just an idea of what its supposed to do

Are you trying to place the door at a valid wall for it? You won't be allowed if there isn't ground at both sides, or if there is an object (window, picture, phone, etc) already in that wall.

I thing you just need ground (floor) on one side of the wall. The game has added flooring on the other side for me in the past.