bolt-python | A framework to build Slack apps using Python | Websocket library

As per the documentation provided :
https://slack.dev/bolt-python/concepts#authenticating-oauth

Bolt for Python will also create a slack/install route, where you can find an Add to Slack button for your app to perform direct installs of your app. If you need any additional authorizations (user tokens) from users inside a team when your app is already installed or a reason to dynamically generate an install URL, you can pass your own custom URL generator to oauth_settings as authorize_url_generator.

On the 'OAuth & Permissions' page of your app, there is a section for 'Redirect URLs'

What it does is that when user installs the app, it redirects to the user implemented endpoint.

OAuth Flow

1. when the user installs the app using 'Add To Slack' button or using https://slack.com/oauth/authorize

   
   Details here : https://api.slack.com/legacy/oauth#authenticating-users-with-oauth__the-oauth-flow__step-1---sending-users-to-authorize-andor-install

2. The generated code is redirected to your specified endpoint - "Redirect URL" https://api.slack.com/legacy/oauth#authenticating-users-with-oauth__the-oauth-flow__step-2---users-are-redirected-to-your-server-with-a-verification-code

3. Solution: Now you need to implement this endpoint to generate access token https://api.slack.com/legacy/oauth#authenticating-users-with-oauth__the-oauth-flow__step-3---exchanging-a-verification-code-for-an-access-token

• After generating token, you can write your code to :
  1. Create a channel
  2. Send user an initial message

I was missing the message.channels permission under the Application Settings > Event Subscriptions > Subscribe Bot to Events.