Mentioned link for chapter 3.2 is to configure the checkmk on GUI, However i am not able to open checkmk site using http://external_ip:nodeport,
I think there is a misunderstanding here. “http://external_ip:nodeport” will not give you a Checkmk site. Its an API endpoint for cluster-collector to which your Checkmk site should connect.
Your Checkmk site URL should be separate. Where have you installed Checkmk ? On-prem ?