So when it says the file cant be found it generally means some element of the path is incorrect.
If you are running this on Linux you may (depending on your OS) need to specify the exact location. You can find this with something like locate psexec.py or which psexec.py or even find (find / -name "psexec.py" 2>/dev/null).
Then if it isnt on your path, you can invoke it by using the full path - for example if it was in /usr/share/some/folder/ you could run it with /usr/share/some/folder/psexec.py
It may also need python first - so some trial and error is needed.