If you already owned halo nightfall and it shows as “buy nightfall” in the halo channel then this may be a solution to the problem. This happens when you own it from the xbox one marketplace instead of the online store…
-Verify if it appears as owned in the xbox web page. if its not, you’ll need to buy it again from the web page. Then you can contact to xbox support to get a refund. After that, go to the halo channel app and it should now be fixed.
Note: Before doing this you may contact the xbox support so the will tell you if youll be able to get the refund.
P.D. My english sucks cuz im mexican, México rules.