In UI 16, not all notifications set up as messaging content can be subscribed to send to a TEAMS channel through the notifications preferences.
There are two types of messaging content: SIMPLE and BUTTONS. See https://docs.servicenow.com/bundle/newyork-servicenow-platform/page/administer/notification/task/configure-message-content.html
Non-actionable messages (=simple messages) are typically sent to a larger number of recipients, and due to rate limits with Teams and / or Slack, those are sent to channels to reduce messaging volume.
Notifications that go to an individual often require an action, such as approvals, accepting or rejecting a solution, etc., which is done with buttons notifications.
Simple messages are not selectable via notification preferences, but only button messages can individually be selected through notification preferences. This behavior is by design.
To make message content available for individual selection against a teams channel notification device in the notification preferences form, ensure the message content type is "buttons" and add a button to the message (eg. OK button). Simple messaging content cannot be individually subscribed to. The error "Not available via this channel" can be seen when hovering over the slider used to associate to the message to the device.