You can only void Approved expenses. If you need to void a request, go to the approved tab by clicking on it or by clicking Approved on the left-hand nav:

Next, on the Approved listing page, click the Void link for the expense:

A modal window will appear. Fill in the reason you are voiding this transaction and click the Save button:

Next, a modal window will appear asking if you are sure you want to void this transaction. If you click yes, the expense will be voided. You should only void an expense if you have returned or did not purchase the the approved item or service.