trunk does not close

You may have moved the trunk latch to the closed position when the trunk lid was open. Try pushing the trunk release button and see if the latch moves to the open position. You may have to manually move the latch open if it does not move by itself.

It probably is a loose latch--go to google--type in the search--how to remove rear trunk liner for slk230. You will find a great step by step "how to". once your have removed the rear trunk liner--tighten the screws. You will need a set of "torx" to fit the screws. It is good to have these tools--as many screws are not longer just flat or philips anymore.