"The front of this card is blank"

Anki displays cards by combining the fields you've entered with a template that says which fields should appear on the front and back of your cards. If you receive a message that a card has a blank front, it means either none of the fields included on your front template have any text in them, or you have fields that have text, but none are included on the front template. You can fix this problem by editing the card on the computer version, clicking on Cards, and checking to make sure at least one field with some text on it is included on the front template. You can add extra fields with the Add Field button.