When I put the code into my steam it said it is already used.
Check your messages on Halo Waypoint, you should have received a new steam key to help with this. Unfortunately, I don’t know if that will solve the problem, as neither of the steam keys I received have worked.
I’m having the same issue, both the codes that were sent to me have says they have been used already 
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