Peter Principle.

The mistake is making the promotion an all in one immediate process.

In this case, the entirety of the tasks proved too much to handle all at once.

She should have been eased into it instead of thrown the keys.

YTA for not giving her option C. "In giving respect to 2 solid years as a cashier, and recognizing that I have may have made a mistake in promoting you that fast without giving you solid support, and your desire for advancement that you deserve... I am going to promote you to probational, provisional manager. The pay will be reduced but not to cashier, and in that time we will properly train you and slowly add each managerial task once you master what we've already taught you."

But one thing you've got to nip in the bud that whatever mental health issue she has, she does not get to call you all sorts of '-ist'. This is your business at stake, and calling you all sorts of names that jeopardize your business will not be tolerated, this is her one pass, and any further instances will get her immediately fired.