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Okay, so I can't seem to summon a single monster in this game. I'm placing the land cards, getting the correct mana but when I click on the creature to summon it the OK button can't be selected. For example, I'll find a creature with 1 colourless and 1 green cost, and I'll have placed on the field 2 green land cards and a water card, and then tapped them all, but the game won't let me summon the creature. As I understand, ANY mana colour can be used to substitute the colourless cost, so what the hell is going on here? I saw an AI player put an 2 colourless cost artifact into play with just two swamps tapped. I don't want to believe the game is broken, but I've looked everywhere and tried many different things and nothing is working.

Are you tapping lands before you start to cast your creature? If you are, your mana is sitting in the mana pool, underneath your life total, sorted by mana. If you wait to tap your mana until after you start casting your creature, it should work a lot easier.