I completed all the missions on Legendary, but Halo Waypoint did not keep track of me beating Shutdown and Midnight on Legendary. So, I went back and beat them on normal but still no Mark VI armor.
> I completed all the missions on Legendary, but Halo Waypoint did not keep track of me beating Shutdown and Midnight on Legendary. So, I went back and beat them on normal but still no Mark VI armor.
You have to re-do them on Legendary.
I beat every mission on legendary and it didn’t unlock for me either 
From the main menu, press start>career>commendations. Go to Player>Campaign.
Whichever ones it doesn’t have as completed on Legendary unfortunately you’ll need to do again. Even though in the mission selection screen it shows you’ve beaten it (this is stored locally), for some reason the Armor unlock is based off of your Commendations which are stored remotely and therefore will not work if the server is down or unavailable (ie not connected to Live).
Dumb, yes, but unfortunately that’s how it is.