What is the exact error that you get?
How long ago did you buy the additional storage?
When your mailbox gets blocked because you ran out of storage, it can take up to 24 hours before the lock gets released. In some extreme cases, this could take up to 48 hours.
If you are still having issues after 48 hours, use the following link to create a support incident via OneDrive (Microsoft Storage is managed via OneDrive even when it applies to your mailbox). Make sure you are logged in;
https://go.microsoft.com/fwlink/p/?LinkId=528676