The door of truth is found in the shower/locker room. The password will not be known until you complete 2 separate timelines. One gives you the clue and the other will give you the code, but you may not know you obtained the code until you get the clue.
The clue is “Twins Birthday”. You will most definitely know what this is when the time comes.
SPOILERS AHEAD – If you were like me and completed the timeline with the twins, but could not remember what the birthday was, then here is is.