Using classic Outlook for Windows in business environments
How long ago did purchase it?
Unfortunately, it takes a little while before the new storage is actually allocated and the account lock removed.
Normally the new storage shows up within 2 hours and any locks (if you were over quota at the time of purchase) are removed within 24 hours. In extreme cases, this can take up to 48 hours.
You can use the "Get Help" button on the following page if things still don't work then.