IAP not working - Services window is blank

Hi, I had received a message saying that I need to reimport the IAP plugin but was not able to do so - after bringing up my project again the next day, my services window is blank now - I can't seem to download the from the Asset Store either (nothing happens when I click on download). The errors I'm getting state "FileNotFoundException: Could not load file or assembly 'UnityEngine.Purchasing..." --- I'm assuming the error has to due with using an outdated plugin, but I can't get to the screen that lets me reimport the IAP plugin. I am using Unity version 5.4.0f3.