Format of reference document number is invalid

What causes this issue?

You have chosen a document type that is relevant for official Argentine
document numbering and for which the official document number is not
automatically generated. That means that you have to enter the official
document number in the reference field in the following format:

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

XXXXANNNNNNNN
XXXX: 4 digit number for the issuing branch
A: Printing character
NNNNNNNN: 8 digit number field

How to fix this error?

Please enter the official document number in the format desribed above.
Please check also that the printing character entered in position 5 is
defined in your customizing settings of the possible printing
characters.

Error message extract from SAP system. Copyright SAP SE.