Mysql does not run in my Cloud9 EC2 environment

It looks like mysql is not yet installed in your cloud9 environment. You can validate this using mysql --version command. If it's not yet there, you can install mysql depending on which distribution you have. You may refer to this link for the instructions specific to mysql.

https://docs.aws.amazon.com/cloud9/latest/user-guide/sample-lamp.html#sample-lamp-install-tools