1st pic is the door hanger used in storage/motion! The door should have a twist lock @ the bottom that goes in the slot. 2nd pic There are 2 twist locks for the door in the roof section (I see 1 in the pic) that go in the upper slots. Do this after setting the door properly in the lower door opening! Helping? 🧐
Put the bottom of the door in the lower threshold (a recessed grove) in the door frame. Move all the curtains to the sides at the upper door area and tilt the upper door (aluminum plate) slots past the pins in the upper door frame. Then twist the plastic pieces horizontal to lock in top of door. Remove in reverse order. It looks like the right side (in photo) guide/ support wire is missing from the door. This wire holds the door to the ceiling in stored position. It looks like you are also missing the locking pin which also secures the bottom of the door on the little aluminum angle piece for travel. I may be wrong on that. The door may just rest static on the little aluminum angle piece. I sold my Fleetwood Niagra a few years ago and it had the same set up. I'm sure the missing parts are readily available.
I cannot tell from the photo (it might be covered by the curtain) you should have the same plastic "T" piece on the top right side of the door as is on the top left side. These two "T"'s lock the top of the door into the door frame.