How To Build a Slackbot in Python on Ubuntu 20.04 error

Hello I am following your steps using Mac os 14 (for environment variables, I have modified the steps where mac is different from ubuntu). I have setup the secret token, bot token and setup tunnel using ngrok. The event channel is verified on the slack page. I run the python app.py, the app starts properly but is unable to handle requests. I have not modified the script in any way. The error I get is

Exception on /slack/events [POST]
Traceback (most recent call last):
  File "/Users/wang.shuochen/.pyenv/versions/3.9.0/lib/python3.9/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/wang.shuochen/.pyenv/versions/3.9.0/lib/python3.9/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/wang.shuochen/.pyenv/versions/3.9.0/lib/python3.9/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/wang.shuochen/.pyenv/versions/3.9.0/lib/python3.9/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/Users/wang.shuochen/.pyenv/versions/3.9.0/lib/python3.9/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/wang.shuochen/.pyenv/versions/3.9.0/lib/python3.9/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/wang.shuochen/.pyenv/versions/3.9.0/lib/python3.9/site-packages/slackeventsapi/server.py", line 102, in event
    self.emitter.emit('error', slack_exception)
  File "/Users/wang.shuochen/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pyee/_base.py", line 111, in emit
    self._emit_handle_potential_error(event, args[0] if args else None)
  File "/Users/wang.shuochen/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pyee/_base.py", line 83, in _emit_handle_potential_error
    raise error
slackeventsapi.server.SlackEventAdapterException: Invalid request signature
127.0.0.1 - - [10/Nov/2020 16:38:23] "POST /slack/events HTTP/1.1" 500 -

Any help is appreciated.