Usually on MB washer tank there are 2 holes:
one for windscreen washer pump,
another one for headlight washer pump.

If your car does not have the headlight washer the pump location at the front is plugged by a rubber gromet.

This fails quite often.

If this si the case for you, just go to the dealer and buy replacement.

I would buy 2 - keep one spare.

Cheers.