I completed the entire campaign on legendary and it prompted the achievement but I did not get the master chief armor. Any help is greatly appreciated.
Judging by your service record, you have not been connected to Xbox Live when you have played much of the campaign.
Unlocking the achievement for completing the game on Legendary difficulty doesn’t give you the Mark VI armour. To unlock the, you need the Legend Commendation which is earned by completing the commendations for each campaign mission. Commendations can only by earned while connected to Xbox Live, so either you were playing offline or you were disconnected from Xbox Live at some point.
From your service record, you still need the commendations for the missions Forerunner, Reclaimer, Infinity, Shutdown, Composer and Midnight. I’m afraid you will have to do these missions again, making sure you are connected to Xbox Live the whole way through each mission.