Spoilers ahead (need help - kind of stuck)

[Jonas]

I have reached the netherbrain at the great hall. There were a lot of enemys but one of my characters was able to reach the netherbrain part which i could climb, but my character just does not want to. Saying "Thats not going to work" and "Might not be the time" when i want to climb it. But the quest is instructing me to do exacly that. Also i am not able to interact with the 3 netherstones in my inventory. Did i just somehow reached an dead end?

[Tax evasion]

There are two "parts" of the Netherbrain's brain stem (or whatever you want to call that).
The first part is right after the courtyard, where you meet the Observer(s), and other various goonsquads. This one you cannot climb.
The second one is after this part, you race against Nautiloid strikes to race to the top of the building, where you'll get a Long Rest plant (or whatever those things are called), and you'll be able to properly climb up. This is also where Gale triggers his quest.
To access this part, simply go to the left side of the original brain stem.