Why is the plugin not getting redirected back and showing some Mautic error?

Please make sure you are using the correct Mautic base URL and connection keys for authorization. Make sure that the redirect URI set in Mautic APP is the same as your WordPress URL ( or you can copy the Redirect URI from the APP setup guide directly ).

Link for App setup Guide:- https://docs.makewebbetter.com/mautic-woocommerce-integration-app-setup-guide/