I completed the Cairo station mission on normal mode and should have gotten the km6 gen 1 armor on halo5 gaurdians but I never received it.
Try going back to MCC, play a multiplayer match, and then go back to Halo 5 and see if the armor is in your collection.
If that doesn’t work, try changing a customization item in MCC (multiplayer skin, emblem, banner, etc), then go back to Halo 5.