How tight is it? On most automatics you have to spin or turn the torque converter until it drops in to the front pump drive. You perhaps better loosen the belhousing and verify it has seated if not sure.

Maybe mercedes is different but I would not know how as that front pump has to coupled. Wait for others to comment. I could be totally wrong. Do you remember seeing the torque converter drop in if you had it out? I have not installed one of their automatics.