Ok, I had a look at the code of the plugin in and that’s actually pretty straight forward. The error you mentioned is returned by the argparse module. So it’s nothing wrong in the script.
Argparse usually tells you which parameters are required and which one exactly is missing.
Can you share the notification command? And you can change the Python script to get some more output out there. You can get a list of all provided arguments with sys.argv[1:].
This looks like one of the 2 variables is not provided or there is a configuration issue.