The engine most likely is idling too low or not smoothly. Check for any vacuum leaks, and see that the EGR valve is not stopped up.
pres the brake until it moves
One possibility is brake(s) locking up.
I had that in a vehicle i used to drive, and they had to replace the whole wiring system inside the transmission
are you pressing the brake as you try to shift?
your problem could be a safety switch located close to the brake pedal. When you apply the brake you should here a small tick
The brake cable(s) are frozen. You need to replace them. Trying to drive the car in that condition will damage what is left of your rear brakes.
If your Mercedes won't shift out of park, you might have an issue with the brake sensor or the brake itself. The brake must be fully depressed in order to shift from park to drive or reverse.
With your foot on the brake, put the transmission in drive, then reverse, drive, then reverse. If you hear a clunking noise, think about replacing the U-joint in the drive shaft with a heavy duty unit from NAPA auto parts. Get one that has a grease fitting so you can service it.
Remove wheel, brake caliper, brake disc, parking brake shoes etc. Drive out broken stud, replace with new stud. Replace above in reverse order.
it will drive but when in reverse it wont move
With the engine running , and the vehicle stopped with your foot on the brake pedal shift the transmission to NEUTRAL and then rotate your selector switch to 4X4 low then put in drive / reverse ( to switch back from 4X4 low to 2 wheel drive , with the engine running , and the vehicle stopped with your foot on the brake pedal , shift to NEUTRAL and then rotate your selector switch to 2 wheel drive then put in drive / reverse )
Bad wheel bearing? Brake pads worn? Dirt/pebble caught between brake pads and rotor? Bad CV joint?