"The BillingStateCode/MailingStateCode field contains 'X', which isn't an available state. Your administrator can add it for you."

You have State and Country picklists enabled in Salesforce, but the value indicated in the error is not defined as a state. To fix this error you can either change the default country setup, or add the missing value to the list of states. 

You can also disable state and country pick lists. Refer to this article to learn more.