The error came during authentication of pubsub subscriber. The error log is misleading and point to a problm with the service account key file. However, the source code to process the logic is as follows:

https://github.com/googleapis/google-auth-library-ruby/blob/master/lib/googleauth/credentials.rb

# Verify that the keyfile argument is a file.

def verify_keyfile_exists! keyfile
    exists = ::File.file? keyfile
    raise "The keyfile '#{keyfile}' is not a valid file." unless exists
end

It means if file is not present, raise the exception.

Resolution

Just check if your key file path is valid.

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