Can't change difficulty

Hello. New player here. I'm level 5 and I can't change difficulty. I am forced by the game to always play on the highest difficulty I have available. Currently that's suicide mission, and I feel completely useless since like 3 difficulty tiers ago. I want to be able to play on like 3-4 and not like on 7 being a leech. Why doesn't the game let me? The war table doesn't show any UI where it normally should be.