I had similar issue with webhook installation on private GCP GKE cluster. You might want to investigate you internal firewall rules to see if port 9443 is open internally inside cluster - See Unable to create a cluster in GKE
I had similar issue with webhook installation on private GCP GKE cluster. You might want to investigate you internal firewall rules to see if port 9443 is open internally inside cluster - See Unable to create a cluster in GKE