If the PARK brake stays on all the time, even if you try to release the parking brake by pulling on the Parking brake switch on the side of the dashboard, then the vehicle may have a failed parking brake component. The parking brake cable may have broken or the module may be defective.
On Mercedes-Benz S-Class W221 the problem will most likely be a failed parking brake module (EPB) which has part number A2214302949. This is a common problem in Mercedes Benz S class, chassis W221, and models such as S320, S350, S400, S550. This problem also affects Mercedes Benz CL-class chassis W216/C216.
The part number for the parking brake module may also be A2214302349 or A2214301649.
It is possible to open up the unit and replace the broken spring inside and the motor which is the two most common failures.