invalid response flask

[aliadhami]

Hello, I followed the instruction but when I run this command and I go to http://my-server-ip:8080, I get "500 Internal Server Error" on the website and I get this error in the terminal :

 * Debug mode: off
INFO:werkzeug: * Running on http://0.0.0.0:8080/ (Press CTRL+C to quit)
ERROR:app:Exception on / [GET]
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.6/dist-packages/flask/app.py", line 1953, in full_dispatch_request
    return self.finalize_request(rv)
  File "/usr/local/lib/python3.6/dist-packages/flask/app.py", line 1968, in finalize_request
    response = self.make_response(rv)
  File "/usr/local/lib/python3.6/dist-packages/flask/app.py", line 2131, in make_response
    " {rv.__class__.__name__}.".format(rv=rv)
TypeError: The view function did not return a valid response. The return type must be a string, dict, tuple, Response instance, or WSGI callable, but it was a RuntimeError.
INFO:werkzeug:89.144.203.168 - - [26/Dec/2022 09:58:56] "GET / HTTP/1.1" 500 -

I also use Ubuntu 18.04, as you suggested. It would be very kind if someone can help me with this issue. thanks!