Try opening the rear door driver side and operate the inside handle.
Transit side door wont lock.
I had to take out the lock to repair and decided to show whats happening when rotary knob gets stuck.
Info and repair ideas.
Video about how you can open stuck sliding door on ford transit mk6 van.
Sometimes when you press the unlock button on the keyfob to unlock the rear of the van only the back doors unlock.
The side door stays locked.
All doors are closed though.
This happened on a 10 plate transit last year more than once.
The first thing to check is to make sure that any side door roller hasn t collapsed it is usually the middle roller that goes this results in the door dropping enough so that the contact pins for the locking motor that connect the wiring from the body to the door don t align any more.
Then lock it all up as normal.
Sliding door lock not opening.
Issues in ford cars door locks and central lock system.
I assume you still have the manual unlock lock turn knob at the inside back of the door opposite end to the handle if so when you turn it clockwise about an 8th of a turn the mechanism should unlock however if it dosent go round with a sharp click try pulling rattling the door handle as you apply force to the knob and the mechanism may spring into place if your lucky had several successfull openings like this.