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 you can't add a quiz to one of your card sets, then it's probably of the Spaced repetition type. All you have to do is convert it to a Double or Single sided set before adding your quiz.