What causes this issue?

In the purchase order, or when entering an invoice, you have entered a
G/L account that has been set up in Customizing as not relevant for
taxes. When you enter an invoice with reference to a purchase order,
you are not permitted to use this type of G/L account, since all
invoice items require a tax code.

System Response

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?

Check your settings in the tax category for the relevant G/L account.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.