On your comparison report for Product Errors you may see ...
Supplier SKU must not be the same as a manufacturer SKU on variable products.
This is because WooCommerce has a single list of all products and the SKU is a unique field. It can not occur twice. Please make the Code or Manufacturer SKU different from the supplier SKU's
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For example if you have a T Shirt in 3 colours
TSHIRT-GREEN
TSHIRT-YELLOW
TSHIRT-BLUE
The Manufacturer SKU MUST be TSHIRT.