Beat campaign, says one mission is incomplete

I just played through the campaign on Legendary with a friend, but for some reason, it says the mission “Shutdown” wasn’t even completed. We have the checkpoints (Alpha, Bravo, etc.) and the missions after it completed with achievements, but not that one. Do we have to re do the WHOLE mission or can we just do Delta over?

Unfortunately, you have to redo the entire mission.

I can’t even load that level at the moment, even the first time it didnt work until the Nth time I tried loading it.

and am I the only or who doesn’t think legendary is very hard?

I played thru the entire game on Legendary. One level did not register and I had to go back and play it again before I got my achievements.