Usually when the serial number is not recognised, either you entered some wrong numbers/letters, or you are using the wrong Toshiba website for the country or region that your laptop was originally purchased in.
For example, if your laptop was purchased outside Europe but you use the Toshiba Europe website, it's serial number will not be recognised. You have to use the appropriate Toshiba website.
Choose the appropriate region/country here: https://www.toshiba.co.jp/worldwide/region/