Someone please help me its not letting me sell my cards to kate
They can't be a part of your deck unless your deck contains 9 or more cards you will be unable to sell a card.
Without having bought credits , you can only sell 1 card a day , either publicly or to kate .
You need to keep 8 cards in your deck all the time you can't sell a card in your deck if it bring the amount of cards lower then that. You also can't sell Ld cards to Kate (or anyone else).