I am using sidechain compression to duck a guitar track so that he vocal cuts through. However I want to audition the extent of the ducking effect but when I solo the guitar of occurs there is no signal coming through from the vocal to the compressor so the ducking doesn’t happen. Can anyone suggest a way for me to listen to the soloed guitar track being ‘ducked’?

the ‘listen’ button’ (available when control room is enabled) is ideal for this.

otherwise temporarily route the vocal to no output (while still sending to the key input of the guitar compressor of course).

Yeah Control Room (CR) Listen Bus is the way.

Set up CR so your project plays properly. Then open CR Mixer. At the bottom, you should see “Listen.” Set the “Listen Dim” to “-∞” (infinity). If you can’t see those controls, then tap the line where it says “Main” and it will expand.

Then, when you hit the big “L” on any track, it will allow you to hear only the output of that track. I almost wept when Steinberg introduced this, 'cos I could finally listen to FX returns just like on a mixing console solo bus!

Or you can just set the vocal track output to “no source”. The signal will still go to the compressor but you won’t hear anything.

Then when you are happy set the output back to where ever you had it before.

Are you using the vocal track proper, or a copy of the vocal track for your SC signal?
If the latter then you can also solo (or even better, use the Solo defeat mechanism) this copy as it should not be routed to the outs.
Otherwise you might try using Prefader send and ensuring that the “Mute Pre-Send when Mute” option in the Preferences is properly configured.