No tax code can be determined for PO item (&1/&2)

What causes this issue?

The system could not determine a tax indicator for the settlement of
material document <(>&<)>V3& / <(>&<)>V4
& for purchasing document <(>&<)>V1& / <(>&<)>V2&.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Maintain the tax indicator in the purchasing document item. In the case
of a purchase order for external service, maintain the tax indicator in
the entry sheet to be settled.
If you maintain different tax indicators in the lines of an entry
sheet, the system cannot settle these lines according to these tax
indicators. It attempts to use the tax indicator from the purchase
order. If you have not maintained a tax indicator in the purchase
order, the system cannot automatically use ERS.

Error message extract from SAP system. Copyright SAP SE.