From the above error message, we understand that you are trying to delete a product that has been added as a sub-product for the group.
We suggest you remove that product from the sub-products of the group and try deleting the product again. Kindly follow the steps mentioned in below article to remove sub-product from group.
Web: How to remove a sub-product from grouping on web?
Mobile: How to remove a sub-product from grouping on Android?