When trying to create a credit memo we received the following error "You must specify a vendor name for the sales tax account."
QuickBooks provides reporting for Taxable Sales and Non-Taxable Sales, therefore the SalesTax Item still must have a reporting agency tied to it.
Go to the Item List and Edit the Sales Tax Item and verify that a Vendor has been assigned to the Tax Item.
The field "TaxAgency (vendor that you collect for)" is blank. The tax agency should be "Avalara".