You're supposing they train you in the beginning to be a cashier.
You need to ask if that is what the situation is, and if you will be returned to the sales floor, and when.
If it's just a temporary thing, suck it up and deal with it.
They will see that you are anxious, which will make you apt to make mistakes, which they obviously don't want. Don't start making mistakes on purpose though, or you'll be fired.
They obviously need floor staff to be crossed trained to cashier, for when times are busy (the holidays are upon us), or if someone doesn't come to work.
If they see after you've trained that this is not something you are good at, and make mistakes, they won't want you cashiering.
However, you need to show them when you are on the sales floor that you do an excellent job there.
Otherwise, if you're not good at cashiering, and you're not good on the sales floor, why would they keep you?
You've already left 2 jobs, if you don't want to burn your bridges, tough it out.
No one like everyone, or everything at their place of work.