You must specify a value for the Other Invoice Number Source

What causes this issue?

You have not specified a valid field for the
<DS:DE.FKKESSII_INVNFLD1>Field for Other Invoice Number Source 1</>
field, or the
<DS:DE.FKKESSII_INVNFLD2>Field for Other Invoice Number Source 2</>
field. The field must be a valid field that exists in the database.

System Response

The system checks if the field that you have specified as other source
exists in the relevant database table.
The system will not be able to create a valid eDocument since the
invoice number is not valid. The system does not save incomplete or
inconsistent Customizing entries to avoid errors during the process to
create and to validate an eDocument.

How to fix this error?

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Procedure for System Administrators

You have to enter a consistent combination for the source(s) of the
invoice number in the SII Document Types Customizing for Spain.
You can ether enter:
a single source or
multiple sources.
You have to enter a consistent combination for the source(s) of the
invoice number if you have specified an other field as source field.
You have to enter the specific value of the field of the database table
that you have specified as source.
For details see the documentation of data elements:
<DS:DE.FKKESSII_INVNSRC1>Invoice Number Source</>,
<DS:DE.FKKESSII_INVNFLD1>Field for Other Invoice Number Source 1</>
<DS:DE.FKKESSII_INVNSEP>Separator of Invoice Number</>
<DS:DE.FKKESSII_INVNSRC2>Second Part of Invoice Number Source</>
<DS:DE.FKKESSII_INVNFLD2>Field for Other Invoice Number Source 2</>

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

Error message extract from SAP system. Copyright SAP SE.