Variation items represent items with various options available using a parent/child relationship. The parent item contains the common information about the item, and the children items contain the unique variations of the item. This relationship means that the flag fields for these item families will need to all be set appropriately for the family to post correctly.
If you see posting errors for your children items, check first in NetSuite to be sure that both the parent item and its children's flag fields are set to "Add/Update" or "Y". Alternatively, if you'd like to post the children as standalone items, make sure your parent item is set to "Post Children as Standalone", or "C".
Next, be sure the parent field is populated on all of your child items in NetSuite.
If your error messages contain errors related to particular product fields, make sure to map them for that category. Note that required product fields differ by both category and storefront.