Matter's Slack Integration requires a channel for Matter to post public kudos and Feedback Friday messages. Due to Matter's Slack permissions, the app can only see public channels in your Slack workspace when going through the Create New Channel flow.
How can I add Matter to a new private channel
You can create a new public channel, select that channel for Matter, and then make it private. If doing so, make sure to only add team members to your channel that you'd like to use Matter with!
How can I add Matter to an existing private channel?
To add Matter to an existing private channel, simply invite Matter to that channel.
You can do it by typing
@MatterBotinto your channel
Clicking on the Invite them button when prompted