Halo Waypoint says you need to have " milestone in 25 career " to release : Military Police HU / RS / CNM
What are milestones in your career?
If you have the Halo Waypoint app for your Xbox 360, load it up and you can see what your current milestone is and what you need to reach your next one. If you don’t have it already, you should be able to find it from the Apps section 
Halo Waypoint is very confusing for me please help me I can not find there