Postcode Verification: Ensure that the postcode or ZIP Code in the customer's address is accurate and that it matches the designated tax zone. An incomplete or incorrect postcode can prevent tax from being applied on opportunities or invoices.

Billing vs. Delivery Address Mismatch: The tax calculation typically defaults to the billing address. If the tax is not applying correctly, you may need to update the Tax Calculation settings to calculate tax based on the delivery address instead. Recalculating the opportunity afterward should apply the correct tax amount.

Products: Ensure that each product has the correct Tax Class assigned. For example, sales items should have their Tax Class set to 'Sales Tax' instead of 'Default.' Incorrect Tax Class settings will lead to errors during tax calculations.

Orders: Orders that include sales items also need to have the appropriate Tax Class set. This ensures that tax is calculated only for taxable items based on their assigned Tax Class.

Tax Calculation Rules: If your tax calculations involve multiple rates, such as state and local taxes, ensure the rates are combined correctly within the system. For instance, do not combine rates manually unless explicitly required; use predefined tax rules to avoid miscalculations.

Regional Tax Updates: The system doesn’t automatically update tax rates for regional changes. To accommodate a new sales tax rate: - Navigate to System Setup > Tax Rates. - Edit the tax rate for the affected region. - Recalculate existing opportunities to apply the new rate, using the Actions > Recalculate option.

Confirm that the region associated with the customer or opportunity corresponds to an active Tax Class configuration in the system. Without this, tax will not calculate even if the address is correct.

Regularly audit your tax rules under System Setup > Tax Rules to ensure accuracy and compliance with your region's tax laws.