I should qualify an earlier post:
I noted we utilize this window for other functions (such as calling external programs, print managers, etc). This will not function using the print button but only the print preview button: print is a direct call using background logic and thus will not utilize this custom code.
That being said, smartforms will be output using the print button.
Test further in your development client: remove print immediately and delete after output and re-print. Go into sp01 and see if something is being output.
I suspect you are doing this in dev and the printer may be locked and/or delete after output is enabled or its printing to the defined location in customizing (to a different location than you are expecting).