You've referenced a form that can't be found.
Form_name is the name of the form that can't be located.
Common solutions are:
Check spelling. Be sure the form name is
spelled correctly in the script. Also, check the list of forms in
the application and be sure the name of the form is spelled
Be sure the form has been saved. When
creating a new form in the application, be sure to save the form
before attaching scripts to items in the form. Otherwise, any
reference to the form in scripts will produce this message.