Hi @antanas,
WPC Frequently Bought Together for WooCommerce is a plugin developed by the WPClever team to help online store owners sell more products in a proficient way and promote cross-selling products across several product lines. Users can add extra items for common product type: simple & variable products or special types such as smart bundles, grouped products, composite products from WPClever developers but can’t add for each variation.
You can read this instruction https://doc.wpclever.net/woobt/ to set up your product correctly
Thank you and best regards,