Unable to delete google app engine flex environment instance

After I was able to ssh via Google web console, I did the following steps to resolve this:

1. Generate ssh key using

   ssh-keygen

2. Copy the key.pub file contents

3. Append the contents to ~/.ssh/authorized_keys file

   sudo nano ~/.ssh/authorized_keys

health checks managed instance groups VS health checks load balancing

The health checks used by managed instance groups are the same health checks used by load balancing, with some differences in behavior. Health checks that you apply to load balancing services help a load balancer determine where to direct network traffic. These health checks do not cause Compute Engine to recreate instances. Health checks that you apply to managed instance groups will proactively signal to the managed instance group to delete and recreate instances if they become UNHEALTHY.

For the majority of scenarios, use separate health checks for load balancing and for monitoring managed instance groups. Health checking for load balancing can and should be more aggressive since these health checks determine whether an instance receives user traffic. Since customers might rely on your services, you want to catch non-responsive instances quickly so you can redirect traffic if necessary. In contrast, health checking for instance groups will cause Compute Engine proactively replace failing instances so you could create health checks that are more conservative than health checks for a load balancer.

https://cloud.google.com/compute/docs/instance-groups/creating-groups-of-managed-instances#monitoring_groups