Great question! This list is probably empty because there are no products created for the beer contained in the vessel you chose in the first step. Have you created a product for the beer/container type (440ml can)?
If not, they can be created from the products section, or for simple beer products like this the easier way can be to:
Click the edit button (pencil icon) next to the beer you want
Scroll to the bottom and tick the 440ml can option under the heading “Automatically create additional products (to sell this beer)”
This will give you a product for the combination of the beer and 1 x 440ml can. When racking you can then rack into this product. If you want to sell the product as multipacks (6 pack, 12 pack etc) as well, then you can create a product for each of the pack sizes, link them to the same beer and your specified quantity of 440ml cans. Creating the multipacks needs to be done in the main products section, rather than from the beer page.
You can also make mixed packs if you need, such as three cans of one beer and three cans of another.
I hope that makes sense! Please let me know if I can help any further.