Chef-solo : Your private key could not be loaded from /etc/chef/client.pem

I installed chef using curl -L https://www.opscode.com/chef/install.sh | bash
But after creating an environment using “knife environment create dev”, when i am executing below command, i am getting an error.

Command: knife environment list

Error: WARNING: No knife configuration file found

            ERROR: Your private key could not be loaded from /etc/chef/client.pem
            Check your configuration file and ensure that your private key is readable

I am commented out client_key and validation_key in .chef/knife.rb. Am i really required knife configuration file? If so how can i get it?

Could you please do the needful help?

Thanks & Regards
Venkat