Process

1.       Go to Help > System Log

2.       Specify the date under Reporting Period. If you are not sure when the Purchase Order was voided, set the Reporting Period to All.

3.       Change the Type from All Types to Void Purchase Order and press the Search button 

In the above image, there are two Purchase Orders that were voided. N0000010 on August 30 and N0000016 on September 27. Pressing the Details will show who voided the purchase order: 

Note: There is no way to see the details of what was on the voided purchase order. That information is not stored.