I know this will sound lame, but I too just got the game, and when I am..

Just hit the B button to exit the menu.

You probably equiped too many high numbered cards. Your CP is probably too high. When you add cards look at your CP. If the left number, your current CP, is higher than your right number, your max CP, it won't let you leave. You have to remove cards so your max CP and current CP are the same or your current CP is less than the max.

Let me explain....first your CP are the numbers under your decks name.
Cards with higher numbers like 5,6,7,8, and 9 take up more CP than 1,2,3,4 would ( 0 is an exception it's value is as much as 9's).
If you have a lot of 9's substitute them for 8's or 7's.
I'm sorry if that explanation was confusing :'(

I hope it helped.

For Sora's version, your card points that you can increase when you level up will determine how many cards you can have in your deck. Each card has a certain amount of points, weak being lowest and strong being highest, with the exception of zero cards because of their abilit. Also, I think different keyblades and spells deal more damage, so that contributes to the card point too. Say you had 100 card point to use up for your deck, and you had 10 cards, each with a point count of 10. That would fill up the deck, therefore you cannot add a card or change a card if it exceeds the amount of points you're allowed to spend. I don't see how you can't remove a acrd, maybe I'm forgetting something, but it's all in the points. In Riku's version you have no control over your deck, which totally sux