global name 'reverse' is not defined

reverse isn't a builtin function in python. Presumably, it's a function in some web framework for doing reverse-routing (getting a url path from a name). The notify_url has to be a url in your application that Paypal will send notices to.

You need to import the function reverse:

For Django 2.0 and up:

from django.urls import reverse

For older Django:

from django.core.urlresolvers import reverse

It's specific to django, but it looks like you're trying to build a URL anyway, so it's probably what you want.

in Django2.0 :

from django.urls import reverse

--Use this code in models.py......

from django.urls import reverse

def get_absolute_url(self):
    return reverse('url-name', kwargs={'pk':self.pk})