Why I can't get the advanced soldier rig?
Solution 1:
When you load a cleared game, you receive a "Level 6 Schematic" in your inventory. Bring this to the Store, and you will be able to purchase the suit.
I don't remember exactly, since it has been quite a while since I played, but I believe that if your inventory is full when you load the game, you won't be able to receive it. It might be dropped at your feet, but that's just speculation.