During the migration with the Battle for Tertium release, your unlocked difficulties were determined by whether you had completed at least 5 missions on the previous difficulty. For example, if you had completed 5 Heresy missions, Damnation would be unlocked automatically.
If you don’t have access to a higher difficulty after the update, it means you hadn’t completed enough missions at that difficulty before the migration to unlock it instantly.
In the Mission Terminal, you’ll see a progress bar below your current difficulty above the ‘Play’ button. Once this bar reaches maximum, the next difficulty will unlock.
To learn more about this, please see: Free Update - Battle For Tertium: Mission Board Rework