Thank you Kent.Meyer for explaining to me that there is a limitation with the application.
Only for a Service Request when a user is logged in as a Self Service user, the "Prompt" function is not available.
The workaround is that in the Service Request - Business Rule - Triggered Action, populate the Notes field based on a condition.
If the Current User Role is Not "Self Service" then prompt for the notes, else hard code the text.
$(if CurrentUserRole() != "Self Service"
then Prompt("Enter Notes for cancellation of Service Request: ", "")
else "Cancelled by Self Service User: " + CurrentUserDisplayName())