This issue means that the field you linked in the Mapping step for 'description', 'size' or 'brand' is empty for a number of items.
How do I solve this issue?
The data is in another field: For example, if the 'size' is empty for some products but you have the size in your titles, you can use a search for value rule to fill in the missing sizes.
The data is not in another field: Add the information via your import file.
