Thank you so much, @Ermin Trevisan and @Niyas Raphy, indeed due to referral integration it is impossible to delete any entity which has a reference in another table

As a amendment to Niyas' answer: such core entities in ERP systems have so many interdependencies with other objects, that it is almost impossible to delete them. Do archive them instead. And never use a productive database for testing or make a backup before.

Hi,

You are getting this error because those records that you are trying to delete is referenced in some another model that is why you are not able to delete it and getting warning like this. 

If you take the example of the product itself, the product you try to delete may be referenced in other tables in the database, like sale order or in product.wishlist  as shown in error message. So first you have to delete those records and then delete the product. 

You can try by creating a new product and click delete button, the record will get deleted instantly as it is not referenced in any other tables. If same product is added to a sale order, then you cannot delete the product unless you delete the sale order first.

Thanks