Amazon Error 99003: Missing a required value...
In this case, you need to:
- open your listing profile
- find the template field size_name using the search box
- click the magnifier icon next to the field
- select your “Size” product attribute
If your shirt comes in different colors as well, do the same for the field color_name where you assign your "Color" attribute.
Please not that if your variation attributes are different from the officially allowed attributes - like "Colour" or "Package Size" - you need to tell WP-Lister which WooCommerce attribute to map to which Amazon attribute on the advanced settings page.