The notification must be saved before this function is executed.

What causes this issue?

Whilst processing this notification, you have executed a function that
has just been performed, or is similar to a function that has just been
performed.

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?

Save the notification and execute the function once more, to avoid
complications.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.