APM Insight monitor was not added to the APM Insight server even after adding the APM Insight agent to my application server with the right credentials

Check if you have restarted/started the Application Server where you have installed the apminsight gem.
If you have restarted/started the Application Server but the monitor still needs to be added, check if you can connect to the Applications Manager server (http://<apm-host>:<apm-port>/) from your browser.
If a proxy server exists between APM Insight gem and the Applications Manager server, specify the Proxy server configuration.

To check if there is a proxy, open a browser in the APM Insight gem installed machine and try to connect to the Applications Manager Server. It should display the login page of Applications Manager. Check whether any proxy server is configured in the browser and specify those values in apminsight.conf.