To create a relationship, both the parent SKU [%s] and the child SKU [%s] require a VariationTheme.
This error occurs when you have not provided enough product data to properly create a variation (parent/child) relationship between products. To create a variation relationship, you must provide a VariationTheme value for both the parent product and the child product. To resolve this error, provide the same VariationTheme value for both your parent product and any child products.
For more information about creating parent/child relationships, please see Creating Parent/Child Variation Relationships.