This happened on a 10 plate transit last year more than once.
Transit van side door wont lock.
Video about how you can open stuck sliding door on ford transit mk6 van.
In my case i could have probably unhooked the yellow.
What happens is when it receives the lock signal because one of the locks wont fully lock as above it then kicks the locks open again as the system thinks there is a door not fully closed etc.
All motors work on all doors as can hear them even clear the contact on the side sliding door still does the same locks then unlocks its self deker gough may 2020.
Try opening the rear door driver side and operate the inside handle.
If not if its the same then there is usually a sticking sometimes faulty lock mechanism on the van.
I had to take out the lock to repair and decided to show whats happening when rotary knob gets stuck.
If your drivers side door won t open try spraying some wd 40 into the area where the door connects with the lock inside the car.
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.