there should be an objective smack dab in the middle of the screen in giant letters that says whether or not you've bought enough packs. The X unlocks when you've accomplished that task.
Oh, that's because you bought packs that weren't "Bronze I", you have to specifically buy "Bronze I". You bought a Bronze II pack or something because you have cards with power of 4 on it. My suggestion is to redo the intro and only buy the first pack 3x. Also to note, don't sell any cards until you've done the objective. It probably messed up the calculations the game is doing.