My guess, and my guess is based on working on a few of these, is that the capacitor you're probing in the pic is bad.
That's the "RUN" Capacitor for both the Compressor and the Fan. The Fan runs so that half of the Run Capacitor is good. Terminals are probably marked FAN (which shows in the pic, brown wire on it), HERM or COMP (for Hermetically Sealed Compressor) and COM for Common which usually is not marked.
Did you wire the SPP6 correctly? It should simply go across the Compressor Run Cap which would be HERM and COM. But if the HERM section of the RUN Capacitor is bad, Compressor still will not start. Should start on Shore Power with a good Start Capacitor (like you SPP6) or without one, but MUST have a good RUN Cap.
