Program error: Event &1 for format &2 in appl. &3 not defined

What causes this issue?

You have the option of changing payment medium lines with a function
module. The lines are released for changing before the append
structure; the changed lines are copied over prior to being written.
To change payment medium lines, you have to enter a suitable function
module in Customizing, which was not done in this case.

System Response

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

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.