Go to Listings -> Unlisted Products and use the filter to display products not listed on your desired sales channel.
*If you do not see the product that you would like to create the listing for, please check the following to be sure that it meets the criteria to be in the Unlisted Products section:
- There is not currently an active listing on that sales channel.
- There is a quantity above 0.
- In the product details -> storefronts tab for that product, check to see that the available to list checkbox is checked.