The separator is missing for the character strings

What causes this issue?

During print editing of serial numbers, the system was unable to find
a separator symbol (string) for separating the individual serial numbers
from one another, or from serial number intervals.
For example, the serial numbers 1, 6, 9 are edited as "1 6 9" by
entering the separator symbol "-". The underscore is interpreted as a
blank.

System Response

The system does not allow further processing.

How to fix this error?

The print editing of serial numbers is used, for example, in the print
programs for printing delivery and sales documents. Check the call of
the print editing of serial numbers in your program. The separator
symbol (string) for print editing is essential. Blanks are created by
using an underscore.

Error message extract from SAP system. Copyright SAP SE.