The 2019 Form 1040X (the amendment Form) will not be available until February 5, 2020.
If you are being audited, I would imagine it was for a prior year which is currently available. You do not need to purchase a State to amend your federal return.
Sometimes "adding a state" is recommended as a work around to help a customer get into the Federal return but the customer is always able to "delete" that state after the return is opened back up.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
