In order for KC to send emails, you must enter your email settings in File, Business Info, Email tab.
If you're using a free OR paid G Suite/Google Workspace account, these are the settings to use.
This information comes from your email provider. If you are unsure of the settings, contact your provider for assistance or do a Google search for them. Example: If I'm using @yahoo.com email, I would do a search for "Yahoo SMTP settings".
Once you have the correct settings, hit the Test email settings button.
If you get the successful message, KC has been configured properly and you are ready to use any email function in the program.
If you get an error, you'll need to allow your Google account to allow third party programs like KC to use their email server.
G Suite/Workspace (paid email account) users, please see this guide: Enabling G Suite to allow third party software to send emails
Once you have enabled less secure apps, please go back to KC and test email settings again. If success, you're good to go. If still error, please see this guide: Error Gmail Still Blocking Account Access After Enabling Less Secure Apps