Cognito - Client is not enabled for OAuth2.0 flows

I've successfully set up an AWS Cognito environment that runs on Localhost following this tutorial.

For the next step, I published the app to my external web server. I confirmed that the Cognito configuration (i.e. Client ID, Metadata Address, Region, etc) is correct. However, when I access and try to sign into the newly published public site I receive the follow error: " Client is not enabled for OAuth2.0 flows."

EDIT:

Here are my App Client Settings

I would like help with what I should look into in the AWS Cognito config or the Server config settings. The server is NOT using a load balancer. I believe the issue may lie somewhere in the Reverse Proxy or HTTPS settings.