Shiref

I have an issue with my transmission. the computer has this transmission fault called gear may be engaged without braking drive moderately, briefly the problem is all when the car stops and the d is engaged the rpm goes higher in the first 5 seconds then drops to 400 or 500 sometimes when i put the R the cars turns off. would somebody help me diagnosing what would be the problem plz as i get alot of different opinions from services and i dont trust them. it seems to be a minor issue but needs an expert to detect it. Thanks in advance