What causes this issue?

You have requested to generate payment advice. However, no line is
selected or active for processing.

System Response

In addition to specifying a non-zero amount for the targeted line code,
the user must actively select the line or lines for processing. The
default behavior is that no line codes are actively selected.

How to fix this error?

Use the line selection button next to the repetitive line code to
select the line or lines you wish to process. Advice will only be
created for those lines which are actively selected on the display.

Procedure for System Administrators

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.