In order to further investigate the issue can you provide the following information:
1. Have you added the SSH key to Project metadata or Instance metadata?If its instance metadata, is
project level ssh key blocked?
2. Does your user account has necessary permission in the project to SSH to the instance (e.g Owner, Editor or Compute Instance Admin IAM role)?
3. Are you able to SSH to the instance using ssh client e.g Putty?If yes, you need to make sure Google
account manager daemon is running on the instance.
Looking forward to your response.
Faizan