PHP - UNGENT!!!

[Rodrigo Padula - Editora Sagra Luzzatto]

Hello!! 
 
I made a form web with PHP that generates a file txt, this he/she calls 
the gnup (gpg) and cryptograf  the file txt and  sends it for email. 
 
 
But I have a possession, I get to generate the file txt with PHP however 
when I call the gpg using the function system () a mistake returns: 2 
 
What does mean that mistake number 2?? 
 
How can I solve that??? 
I need help urgently