This issue because of your mailbox size.
Log into Outlook on the web and clear out old mail - start with deleted items then work on other folders if you can’t get it low enough.
- Empty the recovery folder linked at the top of the Deleted items folder. (The recovery folder has a different, smaller quota, but emptying it can help speed up the processes.)
- Sort the deleted items folder by size and delete the largest messages - select the first, scroll down and hold Shift as you select another message. Then hit Delete.
- Empty the recovery folder again.
- Click the Empty Folder button.
- Go do something else for a couple of hours, more if you have a lot of deleted mail.
Some people prefer to repeat step 2 to clear the Deleted items folder. If you do this, do step 3 every few thousand messages.