I already own Halo nightfall and have watched it but Everytime I go to watch it through the Halo channel it tells me I need to buy it. I then go to the store and it tells me I already have it installed but I still haven’t got the armor set.
Did you originally buy it through the Halo Channel? There is a special version that is available via that app that allows watching it to tie into your Halo 5 game. If you have the general film from the store / movies and TV app, that won’t work.
Now I’m still not 100% on whether the Nightfall promotion is still a thing. The armour itself is available in the General REQ system now so you can find it in Gold packs