The filter for attributes in widgets is not working.
The size filter displays the number of products available in that size, but once selected, it shows a warning that no products are available in that size.
This happens for every attribute: size, color, brand.
Try to regenerate Regenerate the product attributes lookup table in Woocommerce > Status > Tools https://take.ms/LGpvf
In addition, please check Woocommerce > Settings > Product tab > Advanced tab and disable Use the product attributes lookup table for catalog filtering. Also, make sure you added variable products and check again.