ERR_TAX_CALC on QuickBooks: Tax calculation error. Root cause: The tax calculation engine failed to determine the correct tax rate or amount for the transaction. This occurs when the product or service is not mapped to a valid tax code, when the customer's tax exemption certificate has expired, when the tax jurisdiction cannot be determined from the address provided, or when the tax service API is temporarily unavailable. Step 1: Check tax rate. Verify tax rate is correct Step 2: Check tax category. Verify item tax category Step 3: Update tax settings. Correct tax configuration Step 4: Recalculate. Force tax recalculation