I have a big problem despite all my attempts it is always impossible for me to open the cargo bay doors
Hi, thanks to GLS.
He helped me with this info.
So, similar to the real thing, the "Maj Func" switch near the keyboards controls which computer is talking to the associated display: if the switch is in GNC that makes the display show GNC stuff (GPC-1), and the SM position will show SM stuff (GPC-2), which has the Payload Bay Doors software.
The GPCs have a "standby mode" called OPS 0, and currently the SM GPC should be in OPS 0 during launch and landing, as the implementation is still far from the real thing. So, once in orbit switch a display to SM, and it should be in OPS 0, and you type OPS 201 PRO, and it will show the Antenna display, which currently shows nothing but will have some goodies in the future. OPS 202 PRO will bring up the PLBD display, which is the one that is needed to open/close the doors.
In the Post-Insertion checklist there is a block called "DETAILED PLBD OPENING PROCEDURES", which has the (simple) instructions to open the doors, automatically or manually. For closing the doors, it's pretty much the reverse process, which is detailed in the Deorbit Prep checklist.