I recently got the Invalid SKU error. I’m not using WPML but if I create a new product and give it an SKU under Inventory or have an old product that has an SKU already assigned, then add variations, and give each variation a different SKU, the error occurs. I usually assign the first variation the same SKU as the original product.
Once I change the SKU under Inventory to a different number, the error goes away.
Example: Original product SKU is P0001. Variations: P0001 and P0002. This will generate an error.
Change Original product SKU to P0000. Keep Variation SKUs: P0001 and P0002.
No error.
The error did not happen on earlier versions of Woocommerce but I noticed it after I upgraded to the 3+ versions.
This may not be the exact same problem as described by some in this thread but I thought it may help those who have may have the same problem as I did.