Hi Muhammad,

By default you're only able to create project tasks from products which are of type services. You can't automatically generate project tasks from products that are of the type 'stockable' just like you do with products of the type 'services'.
If you want it to work the same way you will need to do custom development in order to make this possible.
For more information have a look at the documentation at https://www.odoo.com/documentation/user/11.0/project/advanced/so_to_task.html

Regards,
Yenthe