"No cards found" for Stroke Order test

I just got Pleco and am using it on an iPhone SE. I created a dozen flashcards from the dictionary and am trying out the flashcard tests. All the other tests (e.g. review only, self-graded) work fine. But when I choose Stroke Order a dialogue box pops up saying "No Cards Found". I can't find a way around this.

Any suggestions? Thanks.