What causes this issue?

The value for field "&v1&" in the interface to Financial Accounting is
an initial value but you are required to make an entry in the field
selection for G/L account "&v3&" in company code "&v2&" linked to the
field selection for posting key "&v4&".

System Response

Error

How to fix this error?

It might be an error in the configuration of the G/L account field
selection. The initial application, used to call up the interface must
otherwise define a value for field "&v1&". If this is the case, contact
the consultant responsible for the application used to call up the
interface or get in contact with SAP directly.

Error message extract from SAP system. Copyright SAP SE.