Yes, try only with the GO app first.
Same here with mine. It comes with android 6 version, before with 5, there was no issue like that.
Check in the settings / applications / default if there is something that would use the USB connection and removed it.
With your RC on, the quad powered, the tablet powered (the app is not running), then connect the USB or disconnect and reconnect it. It should ask what application to use or launch GO app directly (if it is set as the default application). You can also check in the status bar of the tablet if there is an icon "Connected to a USB accessory", on mine it is set to "Charging".
Many report that their USB cable were faulty, so good to try another one.
To use Litchi after using GO app, I have always to clear application default setting for the GO app, connect USB, then it ask what default app to use, select Litchi and then Litchi become the DEFAULT app. To use GO app now it is the same story to clear default etc... So I give up using Litchi on the Tab S2, I use just GO app and Litchi on another tablet.
This seems not to be android 6 related but more Tab S2 related. Some others with android 6 reported correct operation.