I signed into steam one day after the first flight was over (unfortunately, didn’t realized I was chosen to participate) and less than a week ago, a message popped up saying the version of Halo had been revoked from my account. I still have the activation key, I used it to see if I could get it back and it said that the product code cannot be used again.
No steam codes have been released for this weekend’s flight. The code you are trying to use is for an older, closed flight - likely MCC Season 8.
This weekend’s codes and download instructions won’t be set until tomorrow evening at the earliest.