Film back door won't close

The latch is controlled by lifting the rewind knob unit. Looking at the door edge you should see a small metal catch which mates up with a similar one in the body right across from it between the edge of the door seats into. Do you see it the latches (both)?

If yes, carefully look at the body latch as you pull up on the knob do you see it moving? If not the latch is bent or the edge of the camera got banged so the latch is jammed. In either case this will be a bigger job without the correct tools.

If you don't see the latch on the door it came off (spot welded) and you'll need to get a new door unit. On the hinge side of the body you'll need to lift the leatherette to find the screws holding the door.