Hold on, those aren't professions like how you know them in other MMORPGs. Those are Disciples of the Hand and Land, individual classes fully fleshed out just like the battle classes. In other words, they're their own classes, and EXP gained goes to levelling the class you currently are.

If you can't equip gear obtained from Sastasha that means it's for a different class. Look at the 3-letter class abbreviations to see which classes can equip that item. If they're in white, it means your current class can equip it.

I really don't know what's causing those errors for you. All I can say is double check your payment information.