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https://www.reddit.com/r/googlecloud/comments/6vhrgs/permission_denied_publickey/
edit the instance
go to the 'ssh keys' section, and add ssh key from local machine '~/.ssh/id_rsa.pub'. I usually just copy and paste the contents of the file to the web interface. The ssh key will have 'user@host' on the end, edit this to just have the username you require, leave off the
@host portion.
save (you may need to restart also, but try without first).
중요한건 따라 했는데도 안됨 ㅜㅜ