Check your Onyx 12 directory mine is (C: \Onyx12) then open the Server folder and use Notepad++ or Wordpad to open the "AutoServ.cfg" file.
Towards the bottom you should see: ClientProg { "C:\Onyx12\preflight\preflight.exe" }, if the path is empty just copy mine and paste it ( if you have Onyx saved into a D drive change C to D)
As always make sure you backup you .cfg file before making any changes.
It should look like this
NoImgChange { 1 }
ToolbarMask { 5375 }
ClientProg { "C:\Onyx12\preflight\preflight.exe" }
WebProg { "C:\Onyx12\Vendor\jre\bin\javaw.exe" }
WebCmd { " -cp .;.\app.war;..\lib\org.mortbay.jetty.jar;..\lib\org.apache.jasper.jar;..\lib\ant.jar;..\lib\tools.jar;..\lib\javax.servlet.jar;..\lib\cos.jar WebServer" }
Precision { 4 }
NumProcessors { 5 }
MinFreeSpace { 524288000.000000 }