Someone else has used the name you want. Choose something else. Once in-game, you can change your name anytime you like. Just remember that each time you change your gameID you will lose any perk points and score you may have. And in case you don't know, perk points are used to "buy" a ride in a "perk" plane - one that was fairly rare in the war; like the Me262, 4-cannon Corsair, etc.