Hello Rafa,
Guessing that you are doing a VirtueMart product import, you can have a look at
VirtueMart product document to know on how to do product import and the fields available for import. You need to have product_sku and product_parent_sku fields in your import file to create parent child relation with products. To help you more here, you need to post us your sample import file and debug log of the import to check. To know on how to get the debug log, check
Where to get debug information? section on top of this page.
Kind regards,
Tharuna
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