Hello,
Welcome to the Microsoft Community. Thank you for supporting Microsoft products.
I understand the inconvenience of having purchased OneDrive storage but not being able to use it in OneNote.
Here are some of my thoughts:
Some users may have multiple Microsoft accounts. First, make sure that the account from which you purchased OneDrive storage is the same account as your OneNote account.
Then, visit OneDrive.live.com and make sure that the OneDrive storage is active and showing 100GB. if everything is fine in OneDrive, it could be that OneNote has not yet detected the change in storage space and you can try logging out of your account in OneNote and then logging back in to see how it goes. As a troubleshooter, you can also temporarily visit OneNote.com using your browser or access the OneNote app on a different device like your mobile phone and see if you can use your space here.
I hope the above information will help.
Best regards,
Tina L - MSFT | Microsoft Community Support Specialist