Cant equip the classic doom guy armor

I used 2 sentinel batteries on the doomguy skin, but when i go to the main menu to customize, the skin doesnt appear, i have restarted the game several times but nothing is happening

Double check your sentinel batteries and the door to unlock it. I had a similar thing happen with the Praetor suit.

Unlock the skin and start the next mission.  The game doesn't always save progress within the hud till you start a new mission.