When it is the main phase of your turn and you activate the 5 mana activated ability of Urza, it only lets you cast creature spells and no other types of cards. I am unable to cast artifacts or enchantments. I understand why i can't cast an instant card or sorcery card with no valid target, but it's very frustrating to have pulled a great card I need to use and I can't cast it because of a glitch in the game.