To enable Channel Notifications for a private Slack channel, you'll need to add the Tettra app. You can do this in 2 places:
If you've just created a new private Slack channel, click the "Add an app" button, search for Tettra, and add it to the channel:
If the channel has already been created, no need to scroll back to the top! You can add apps via the Channel Details:
Once you've added the Tettra app, you can connect the private channel to a Tettra category by following the instructions in this article.