TalentCards provides you with three types of card sets:
You can add a quiz to Double and Single sided sets, but you cannot add a quiz to Spaced repetition card sets. Also, you cannot convert a card set to the Spaced repetition type if it contains a quiz.
So, if your card set contains a quiz, the option to convert it to the Spaced repetition type is disabled. To be able to do the switch, you have to delete every quiz contained in your card set.