Hai,

from odoo import api, fields, models
from odoo.exceptions import UserError


class VendorValidate(models.Model):
    _inherit = 'purchase.order'
   
    @api.multi
    def _add_supplier_to_product(self) :
        vendor_list=[]
        for order in self:
            for line in order.order_line:
                if order.partner_id.id not in line.product_id.variant_seller_ids.mapped('name.id'):
                    vendor_list.append(line.product_id.name)       
            else:
                 if vendor_list:
                    product_names = ', '.join(vendor_list)
                    raise UserError("Please make sure "+"["+product_names+"]"+" has vendor "+"["+order.partner_id.name+"]")             
 
                I think it works for u..

                Check.

Thanks