Just add

def action_wait(self, cr, uid, ids, context=None):
context = context or {}
for order in self.browse(cr, uid, ids):
    if not order.order_line:
        raise osv.except_osv(_('Error!'),_('You cannot confirm a sales order which has no line.'))
    # Raising exception to inform the user about the stock level before confirming the sale order
    for product in order.order_line:
        if (product.product_uom_qty > product.product_id.available_qty):
            raise osv.except_osv(_('Not enough stock !'), _('You requested %.2f.The available stock is %s !') %(product.product_uom_qty, product.product_id.available_qty))

in sale.py. The def action_wait is already defined. just add the code to into it. My problem got solved.

There is a module called warning. Please check it

Warning messages can be displayed for objects like sale order, purchase order, picking and invoice. The message is triggered by the form's onchange event.

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