This module will likely defines the channel where you wanna send the message when a user leaves from your server, this module are optional like the welcome channel and the moderation channel ones.
To set the channel where Yuki should send a welcome message just type
y!config edit welcomeChannel <yourchannelname/yourchannelid>
No need to worry, if you doesn't want to edit the message, you can just leave it as the default value. But you can change the message by typing
y!config edit leaveMessage Goodbye %username% (%mention%)!