The Auto generated message states- Halo Support is unable to create new flight invitations or modify existing invitations. Hopefully the mods are able to send out new codes for the MANY people that are receiving the Duplicate steam key error. Otherwise this is depresso espresso 
I’m getting this too
There was an issue with duplicate/invalid Steam keys earlier, resulting in codes having to be re-generated and published a few times. 343 have resolved this issue now and distributed new and working codes to everyone affected. If for any reason your new code still does not work, please file a support ticket on the Insider Support Site.
You can check for your new Steam key in one of these three places:
- Waypoint Beta (My Flights): https://beta.halowaypoint.com/halo-insider/flights… - Waypoint Message: https://halowaypoint.com/en-us/messages - Halo Insider (“My Flights” at the bottom): https://halowaypoint.com/en-us/halo-insider/profile