2011 Citroen C4 semi automatic wote start and pops up with gear box fault and when it is running it wote let me change to reverse

You need to scan it for fault codes and rectify as needed

Mine did the same the other day. gearbox fault light. it remained in neutral what ever i did with the shift. I was towed home by AA. I took the cover off the shift handle. Cleaned it with electrical circuit cleaner spray and a powerful vacuum hose and it it started working normally.
fingers crossed it is still working.