Sorry, user xiaonannan is not allowed to execute '/bin/su' as root on engine-1.

As an AI language model, I don't have the ability to grant or revoke access privileges to execute commands on a machine. It seems like you are trying to switch to the root user using the 'su' command, but you don't have the necessary permissions to do so. You may need to either log in as the root user or use the 'sudo' command with appropriate privileges to execute the desired command.