The HPS is a solitary person. And artists/painters are in a solitary business. You work alone, and don't get to socialize. Unlike the Hermit, who isolates himself to investigate the world, the HPS isolates herself so she can focus on, and tap into her intuition, unconscious and, yes, creativity. Anything from the outside would distract her from this.
Maybe you like the idea of this, but you asked the cards why you should not be one, and they may be telling you that it's hard to be isolated with your thoughts and feelings and working alone all the time. And it's especially hard because you don't get much feedback on your work. In fact, others may not understand what you're doing or why. So all you can do, day after day, is focus on painting, believing in it above friends, family, society. So. Even if you have the money, time and inclination to do it, you may find this part of being an artist/painter difficult and unpleasant. Even great artists have gone a little crazy being so much by themselves with only their canvasses.
As an artist (painter) myself, I would second
every single word Thirteen said. It is, in addition, a very mysterious business to break into - seemingly not great artists break through and sell, while many very good artists remain untapped, unrecognised. And not for want of trying to break through. It really is a "right place at the right time with the right people" kind of job to succeed at.
It's also a career that is so hit and miss, with no pay in between commissions. So you can't actually "make a living" from painting, unless you hit that sweet spot with the right place, time and people that I mentioned. You'd really have to beat huge odds.
It can take many long years of training with successful masters in their studios, as well, after art studies in university. And that doesn't come cheap, and it's not paid for by anyone else but yourself.
I think what HP is saying here is that it is the kind of work you can do on the sidelines, alone, just like religion or praying in the temple, but not something you can make a career of, and get paid for while you learn to do it. After all, even in the most literal sense, being a High Priestess is a calling and a vocation, not a career or a job - much like being a painter. And as a High Priestess, one would be sustained by the abbey, and live in poverty. So I think that might also be what she is trying to point out here. The life of the lonely artist in the garret, living in poverty, overlooked by society, is a stereotype because it is TRUE.
If this is your highest calling (being a painter) be prepared to live in poverty, and be sustained with welfare checks by the society that supports you, until someone decides you are the ultimate Delphic oracle of Art, and becomes your patron. And I can only wish you luck with that. It is not a career to enter into lightly, and I think HP is telling you that, as well. It will cost you everything you have; you will have to give up everything fun and material, in order to purchase your supplies, discipline yourself to work long hours daily, alone, and lug your work around from gallery to gallery, hoping for a break. It *has* to be a calling and a vocation, or you will never survive the challenges/commitment it presents.
I hate to be a downer here, but I can tell you from my experience, and that of my friends, that it is a very, very
very hard row to hoe, and you'd need all of the mysteries of the universe conspiring with you to find success - even just enough success to make a living from it (if you want to just use it (art talent) to put paint on canvasses with brushes, that is).
ETA to sum up, High Priestess in this position says: Sacrifice. Commitment. Vocation. Poverty. Relying on the mystery of the conspiring time/place/chance Universe to succeed.
ETA x 2- Since HP is in the position for why you
shouldn't do that as a career, I can only assume the cards were telling you you would have to have the High Priestess level of vow to poverty, hardship, and commitment and vocation to follow through, which I think the cards were saying "that's not you, that won't feed you or nurture you, that is harsh, and that's no fun - so think very long and hard on this before making it your choice".