This error is saying that the product.sku is required. When an error is saying that a field is required, we can assume that the value of this key is not being retrieved from the get method. In this case the key is product.sku.
Let's take a look in the Data Mapping.
Up next search for the product.variant.sku on the send side.
Here we can see that there is no value being mapped against the product.variant.sku.
We need to get the correct key from the get and match it against the product.variant.sku key, like so (the key that we need to fill in can vary from application to application):