If the tax cannot be calculated (due to an error when using NetSuite to calculate the tax), how we handle depends on the underlaying NetSuite tax methodology.
Legacy NetSuite Tax Account
This will assume all items are taxable and whether it is taxable or not will depend on the Customer record.
SuiteTax NetSuite Account
We assume all items are not taxable and write the transaction with the "taxdetailsoverride" flag set. This can be later queried/reported on to handle adjustments manually according to your accounting procedures.