Sources of external programs cannot be displayed

What causes this issue?

You asked the system to display the source code of a program that is
not part of the SAP System. The background processing system can only
display the source code of programs written in ABAP/4.

System Response

No action taken.

How to fix this error?

To display the program's source code, you must use an editor in your
host system.

Error message extract from SAP system. Copyright SAP SE.