What does “Contains cards you do not own” mean?

It is literally what it says; the deck has cards you don’t own yet and therefore, unplayable. There are two scenarios where you’ll encounter this. The first one is when you import another player’s deck via QR code or the shareable clipboard code. If the deck you imported has cards that you haven’t pulled yet, you’ll encounter this error.

Next is when you reaped a card that’s previously part of your deck. This is most likely to happen if you decided to reap normal copies of a card in favor of including the golden ones in the deck, without updating the deck’s card list.

To resolve this, you have to enter the Deck Editor and look for the blue-tinted card on your deck’s card list, on the right-side of the screen. Simply remove said card and replace it with another one. The replacement could be another version of the same card or a placeholder. Either way, you have to make sure that the invalid card is removed from the list.