Channels field in Lightning Knowledge

1. Switch to the Lightning Experience.
2. Click the Gear icon in the top right corner and choose Setup
3. Go to 'Object Manager.'
4. Select the Knowledge object.

Note: If you migrated from Classic Knowledge to Lightning Knowledge, this may be named something other than 'Knowledge'.

5. Go to Knowledge page layouts and edit the page layout in question.
6. Drag and drop the Channel fields to the desired section of the page. The fields are: 'Visible to Partner,' 'Visible in Public Knowledge Base,' 'Visible in Internal app,' and 'Visible to Customer.'
 

For more information see the "Channel" section of the SOAP API Developer Guide - Articles
 

7. Save the page.
8. The 'Channels' field should now be visible when creating or editing the article.