The location you are trying to delete is referenced to other records. that is why you get that error.

For example. in your case

1) you created one location and put a product in that location. at that time odoo creates a stock.picking record that refers this location as the destination location.

2) you replace the product from your location to another location, at that time also there creates a stock.picking record.

so the reference of this location still there in some records.

you think on the way that you can delete that location if there is no product in it. that's wrong. the system already create some records refer to this location. so you can't delete the location till you delete that records.

Hope you can understand.

Thank you.