ERROR: (gcloud.compute.ssh) Could not fetch resource: - Insufficient Permission

It looks like the attempt to SSH is recognising the instance in your project, but the user doesn't have the required permissions to access the machine.

Have you tried running:

gcloud auth login

and completing the web-based authorization to ensure you are attempting to access the machine as the correct (authenticated) user? This process ensures the Cloud SDK you are running inherits the permissions of the user specified in the web-based authorisation. See here for more information on this.

It's also worth adding the link to the tutorial you are following to your question.

Besides the accepted answer, be sure you are in the correct gcloud project

gcloud projects list

Then

gcloud config set project <your-project>