Specify tax codes everywhere or enter '**'

What causes this issue?

A tax code was specified in some of the receivable/payable line items,
and in others it was left as '**' or not entered at all. Furthermore,
there are receivable/payable line items in the document that do not
have a discount base amount. This amount should therefore be determined
by the system (even if the cash discount rate 1 is zero).
This will cause problems when the system assigns the receivable/payable
line items to the individual offsetting entries that are relevant for
cash discount.

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?

Enter your receivable/payable line items consistently by either
specifying '**' (or no entry) as the tax code in all the line items or
entering an explicit tax code in every line item.
If you cannot enter them consistently - for example, because some of
the reconciliation accounts are tax-relevant and others are not, which
means that a code cannot be specified in every item - you have to make
an explicit entry in the "Discount base" field in every
receivable/payable line item (the entry may have to be zero).

Error message extract from SAP system. Copyright SAP SE.