Chromebooks, by default, come with a 100GB of free space. But, they are not automatically assigned to you after your purchase. You have to go to the Google Chromebook Special Offers website and “redeem” the offer.
Here are the exact steps you need to take to get 100GB of free Google Drive space on a Chromebook:
1) Open Google Chrome
2) On the URL field type in https://www.google.com/chromebook/offers/ and visit the website that features “Special offers for Chromebook”.
3) Scroll down to the bottom left where it says “Get Google One with 100 GB of storage and other exclusive benefits for 12 months when you purchase a new Chromebook or Pixelbook”.
4) Click on “Redeem”. And,Voilà! you’re done.
There you have it. Note that this “Special Offers for Chromebook” page also lets you redeem other special promotions that are applicable to your Chromebook.