Why he couldnt learn how to use mana from sol?

I think because Sol has royal duties from now on? That’s what I think and he’s been coming back to Brian after the session is done, too so probably Sol has gotten more responsibilities

Sol doesn't have the power that he used to to have because he had to give them up in order to reside on earth permanently, so lorcan is currently the only being with enough mana to be able to teach Brian what he wants to learn.