#Don't understand where to write this code. — Page 1

Please explain where need to write this code. I have re-read the documentation on this code more than a dozen times and still did not understand. When I try to write this code in files that are responsible for events, I get an error:

node:events:497
 throw er; // Unhandled 'error' event
 ^

ReferenceError: client is not defined

A similar error occurred when trying to insert this code into the command itself.