Hi,

Try refer the following code.

from odoo import models, api, exceptions

class SaleOrder(models.Model):
    _inherit = 'sale.order'

    @api.multi
    def action_confirm(self):
        for line in self.order_line:
            product_qty_available = line.product_id.qty_available
            if product_qty_available < line.product_uom_qty:
                raise exceptions.Warning('Not enough stock available for product %s. Available quantity: %s' % (line.product_id.name, product_qty_available))
        return super(SaleOrder, self).action_confirm()

Regards