The driver must step on the brake to release the transmission from the
Park position. Safety interlock.

Another possibility is spillage on the center console... which would render
the tranny useless.

This is how my problem started (E320 CDi 1999 uk).

Pencil in secret hole gets you going again.

.. but if the gear selector indicator on the dash is not showing true
gear or any gear you may have to go to D drive to get it working
again.

.. you may find car is placed in LIMP MODE ..it wont get out of first
gear and do over 3000 revs .... this could indicate one of several
problems MB dealer will provide printout of stored faults.

Next thing to try is getting someone to spray the brake pedal switch
with contact cleaner .... this has worked for some (and worth doing
anyway).

Changing the battery may also help (did with mine for a day!)

The thing that sort of worked for me was MB changed the Air Mass
Sensor, this has reduced the problem to now and then, and we are
watching it.

Tell me: Does the cruise control work? Does it work when you first
start the car?Try it at several high speeds above say 55mph does it
hold? ...or does it drop out?

I have the above CC problem and still investigating