A Microsoft file hosting and synchronization service.
Sorry to hear that you’re having trouble with this situation. Could you please share a bit more information about what happens when you try to purchase the additional storage? Do you receive any error message? If so, please share a screenshot of the error so I can better understand what is happening.
Also, please let me know how you are trying to make the purchase. Are you purchasing the additional storage directly from Microsoft, or through a third-party provider?
Additionally, please sign in to the following page and let me know what subscription or storage information you see there: