FWIW, on my 1991 Cavalier, if the battery voltage was low (winter time, lots of current draw, or battery on the way out) the thing would go into limp mode on start up. Very often, switching off and restarting would clear the error.
Have you checked the wiring diagrams? Often these are good at giving clues. If it's just a standard auto box then I would hope it would be something relatively simple.
Might be a faulty or loose connection to a sensor or might be telling you the pressure in the box is low. Oil level in the box OK?
Best to get the fault codes read (local independent garage) hopefully pointing you in the right direction.
Good luck selling an auto box vehicle with the warning light on, its just not going to happen, unless its a trade sale at reduced value. Sorry but that's just reality ........
Personally I'd steer clear of any French vehicle with auto transmission......they are the most troublesome of all systems. The lamp often indicates oil pressure problems within the gearbox.
John