You must have tanks in stock on the same chassis as you used to create Marder or Wespe. If you designed the Marder on the Pz II C chassis, the old tanks in stock must have the same Pz II C chassis.
Of course, you must also remember that these Pz II C tanks must be marked as obsolete (turn off the "Decommission" button in the list of tank designs in Production).
Otherwise, the conversation button in the Marder project will be inactive.