Connor Lancaster

Does anyone know why I can't get the achievement for completing the last mission on legendary on single player or co-op and yes live is on and i have completed the other levels on live.

It says your gamertag does not exsist

i beat co op campaign on legendary waypoint says i have twice now, but i am unable to get an achievement and the console says i haven't please tell me how i may be able to fix this

i beat co op campaign on legendary waypoint says i have twice now, but i am unable to get an achievement and the console says i haven't please tell me how i may be able to fix this