As stated above, I’m unable to unlock penitent difficulty. I’m a returning player who is trying to play the vanilla campaign through again on spiritborn on penitent. I only have access to expert. I’m currently a level 21 Spiritborn on this character, but i have several older ones. I CAN select penitent if i start a new character with the campaign skip, but that doesn’t solve what I’m trying to do. i’ve browsed the forums, i’ve checked YT, Reddit, etc. Everything coming out says the same thing. Complete the campaign. Which I’ve completed back on launch and all.
Am I stuck just playing through the vanilla campaign on expert?
Yes. You are not able to play the main campaign higher then expert. But the VoH content is playable in higher difficultys. The restriction occurs only to the normal campaign. This is also written in the description of the difficulty notes ingame btw. 
in game i’m still only seeing Complete the Original Campaign. Yet, my friend is also doing the same thing and waiting on me to catch up. He’s currently in penitent so something doesn’t add up. In his words, it just unlocked for him but no idea what triggered it. He’s level 25. I would like to add that he has completed VoH on his 1st character whereas I have not. Do you think that could be the reason why?