Each tax you've set up in Billings Pro must be mapped to the corresponding one in Quickbooks. At a minimum, you must map your default tax by selecting the appropriate tax name in QuickBooks. If your customers in Quickbooks are taxable, select the "All customers are taxable in Quickbooks" checkbox.
Working from Quickbooks, you must choose Quickbooks > Preferences > Sales Tax, and ensure that the"Customers are charged sales tax" checkbox is set appropriately. If the Quickbooks settings and Billings Pro settings for charging taxes do not match, then you will not be able to export.