Sometimes the translator is having trouble adding new products to your WooCommerce store. One reason for this is you have a duplicated SKU somewhere else. Products should only be added to the web store from Retail Express so maybe something was deleted?
If so the SKU’s from before are now preventing the system from creating the products SKUs now.
The old products you may not see from the front end but they might be lurking in your trash.
Please make sure that you have empty your Product’s trash (bin). If the product is still in your trash, you won’t be able to create the same product id and variation skus. Emptying your trash should allow you to add the variation sku.