In PDQ Deploy 1.2x using a mapped drive for your Installer File path was fine (as long as you, the console user, could access the mapped drive). However in PDQ Deploy 1.4 or higher mapped drives should not be used and should be replaced with a UNC.
When a deployment is initiated PDQ Deploy 1.4+ will use the PDQ Deploy service (AKA background service) to access the Installer file. Service accounts do not have access to the mapped drives that may exist on your console machine. This is true even if the credentials running the background service are the same credentials you use to run the PDQ Deploy Console.
You may notice that when you select an Installer file in PDQ Deploy 1.5+ by using a mapped drive the drive mapping is replaced by a UNC. A drive mapping is kept if you are importing an installer from an earlier version of PDQ Deploy. If you upgrade to 1.5 from 1.2x your drive mappings will not be changed in your existing Installers. These values need to be changed.
This screenshot shows an Installer that would fail because it was using a drive mapping.
Below is a screenshot of the error (and the diagnose window) that you would receive since the mapped drive didn't exist for the PDQ Deploy service. The error, "Not Found - Failed to copy directory to target" is literally true since the Y: drive didn't exist for the service user.
Here is the same Installer but the mapped drive is replaced with the appropriate UNC path.
Using the proper UNC path (and, of course, the appropriate command line parameters...ahem) this installer was successfully deployed.