Before you can list a product you need to create a Listing Template for that sales channel.
click on Listings.
Click on the Sales Channel you want to list your product on.
Click on List Products.
Now you can choose your Listing Template from the drop down box or you can search for the template using the search bar.
Now you can select the products you want to List by clicking the tick-box next to the product. When you have select the product click List Now.
Congratulations you have successfully Listed a Product on your Sales Channel. If there is something wrong, an error message will appear and you might need to make a adjustment to something.