By default, you cannot send Rattle messages to a private Slack channel. But follow the steps below to start sending alerts to private channels.
Steps
Go to your private channel where you want to send Rattle notifications
Enter “@rattle” in the message box
Click the “Invite Them” button to add the Rattle app to your private channel
Go to your Rattle workflow screen (https://app.gorattle.com/workflows)
Configure your rattle and then go to the "Who will get the message" section. Here, click the refresh button to reflect this new private channel in your dropdown list.
Select this private channel to send alerts here.