Prometheus on GCP Kubernetes cluster error "no such host"

I am trying to setup Prometheus on GCP Kubernetes cluster. I followed a tutorial
I have the deployment pod running.
Now I am trying to access the prometheus dashboard by port forwarding as shown in the blog.
But the page is inaccessible.
I tried to debug and checked the logs of the deployment pod and found that there is an error there.

ts=2021-12-31T06:49:42.109Z caller=notifier.go:526 level=error component=notifier alertmanager=http://alertmanager.monitoring.svc:9093/api/v2/alerts count=1 msg="Error sending alert" err="Post "http://alertmanager.monitoring.svc:9093/api/v2/alerts": dial tcp: lookup alertmanager.monitoring.svc on <IP>:53: no such host"


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