data = tornado.escape.json_decode(self.request.body)
来源：https://stackoverflow.com/questions/60920523/python-tornado-request-json
The data you receive in your Tornado server application depends on how your data is being "packaged" by the client. You could have application/x-www-form-urlencoded data (seems to be your case above) which you can process in Tornado using get_body_argument:
drzava = self.get_body_argument("drzava", None)
kategorija = self.get_body_argument("kategorija", None)