Error when making a custom deck

Hi, I've repeatedly had this error whenever I finish making a custom deck, does anyone know what causes it and how to fix it?
The deck, when viewed in Deck Manager, will have upwards of 300+ cards in it with at least 250 of a basic land. If used by me or the AI in this condition, they will have 67 cards in their deck counting their opening hand.
The deck, when viewed in Deck Manager, will have upwards of 300+ cards in it with at least 250 of a basic land. If used by me or the AI in this condition, they will have 67 cards in their deck counting their opening hand.