Hi I've found a door near Monsignor plaza in a building on the second floor says it needs a terminal to open it but I can't find the terminal anywhere any help?

This kind of doors are often used to prevent you going somewhere without the quest planned for it, and it also may be just a 'unidirectional door', which means the terminal to open it is on the inside. In this case you need to enter the building or the area through another door, but can it open from the inside to get out again.

You can try looking for a red cable going from the door to a terminal. If it's the door I think it is, it's actually inside a cobbled-together shack and there is a terminal on a table some distance away from the door.

If its labeled "Ticonderoga", like the one I found near there, it is a Railroad Safe house. Its on their chalk board of safe house locations in the HQ. I haven't sided with the railroad so I'm guessing its a quest from them and a terminal may not spawn unless its active.

Yep that's Ticonderoga. It's only accessible after you do a certain Railroad quest.