Invalid variation relationship: The child variation is already assigned to a parent SKU. Please delete the old relationship before submitting a new relationship.
This error code only applies to variation relationships. It is returned when the child in the variation relationship already has another SKU stored as its parent. You must first delete the old relationship before submitting the new one.
To eliminate a relationship, you can delete either the parent or child product. When you delete a parent, you remove the relationship between that parent and all its children. When you delete a child product, you eliminate the relationship between just that one child product and its parent.
After you remove the original relationship, you can re-assign the child product to a new parent product. See Creating Product Relationships.