does the gear shifter actually move into D?
do you get the gear indicator display on the dash showing you are in D or is there a F present?
is there no drive at all from the vehicle?
Best regards, Paul
F and no drive
The F means a fault will be stored in the transmission control unit to help pinpoint the problem so the first step is to have the vehicle checked on a diagnostic tool such as Mercedes STAR to check to see what codes are stored.
Common problems can be with the gear selector module or the valve body inside the gearbox but there are many other possibilities.
The oil level should also be checked with a workshop dipstick (as no dipstick is installed from the factory).
There can also be mechanical problems inside the gearbox.
Once the codes are read out it should help pinpoint the problem.
Let me know if you need anything else.