There are a number of different errors that the Quality check will show. One of the errors you can encounter has the format 'description'/'size'/'brand' is empty.
If this error is shown it means that the field you linked in the Finalize-step is empty for a number of products. You can click on the error to see for which items the field is empty.
How to solve this error depends on the missing information and if the information is available in another field of your items. For example, if the size is missing for some products but you have always included the size in your titles, you can use a search for value rule to fill in the missing sizes. However, if the information is not available in another you will have to add the information in your import file.