It has taken me a while to figure out why I got this particular error (displaying in red at the top right corner under my profile image in Heroku) since I did not find any relevant info in Google search.

The error means that there is an issue with an authorization specifically on your Github account (this could be due to, I guess, inactivity in the deployment of GitHub projects linked with Heroku).

The solution is pretty straight forward:
(1) Click on the Deploy section in Heroku
(2) Scroll to App connected to GitHub
(3) Click on the Disconnect... button
(4) Allow the authorization in the connection between Github and Heroku -> Enter your Github password
(5) Reconnect to your Github project
(6) Then you should be able to deploy successfully in the Manual deploy or Automatic deploys sections!!

Hope this helps if you also encounter the similar issue! :)