I completed the halo 4 campaign on legendary and got the achievement but did not get the MJOLNIR Powered Assault Armor/Mark IV could you please help ?
Check to see which levels the game thinks you completed. It will display it in the mission select screen of the campaign.
I checked over your service record - currently it’s only showing that you have completed the first 2 missions solo legendary and mission 3 legendary co-op. If you were offline while doing the legendary run, it won’t count unfortunately as it needs to be online completion. If you’re stuck, have a look at some of the legendary guides from MythicTyrant or NakedEli on Youtube. They have some great strategies to help make sections a bit easier 
To complete the commendation you need to have xbox live connection in the level.
If you lose xbox live any time in the mission the game says you have failed to complete the commendation to avoid cheating.