Dear Ahmed

Check This Function and Make Sure didn't change or Oveeride by correct way:

    @api.model
    def create(self, vals):
        if vals.get('name', 'New') == 'New':
            vals['name'] = self.env['ir.sequence'].next_by_code('purchase.order') or '/'
        return super(PurchaseOrder, self).create(vals)

Also make sure the name field Exist on the xml file

I hope I helped You...