I'm trying to update Inventory product on hand quantity automatically from my module.

Basically, I search the same name in inventory

inventory_product_id = self.env['product.template'].search([('name','=',product.product_name)])

and creating new record for stock.product.lot as new serial number and after trying to write on hand quantity +1

lot_object = production_lot.search([('name','=',product.product_barcode)])
stock_obj = self.env['stock.quant']
stockvalues = {
    'product_id': inventory_product_id.id,
    'location_id': 8,
    'lot_id':lot_object.id,
    'inventory_quantity':1,
}
stock_obj.create(stockvalues)

But, when I run this script i get message:
Sorry, you are not allowed to create documents of type 'Quants' (stock.quant).
No group currently allows this operation. - (Operation: create, User: 2)
When I trying do the same as superuser, the script works but still doesn't put any numbers except 0 to on hand quantity.
Does somebody know some work around or different approaches to change on hand product quantity automatically in Inventory module?