Make sure that the Customer default file is set up. 

The customer default file should be set to default the proper information into the Taxtype field of new customer records.  In normal circumstances, a Taxtype of RETAIL is automatically loaded into new customer records, and the majority of users are restricted from changing it.  Once a resale certificate is received for the customer or shipping address, the accounting personnel who have rights to change the Taxtype field can modify the information (to resale in this case). This allows sales to set up new customer accounts while still allowing the accounting department to control the sales tax information.

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Sales Tax System Startup Checklist