We don't really see much documentation on these features. On our laptop, we connected the HDMI cable to the surface to output the video then, based on the little documentation that we found, connected a USB-A 3.0 (on the laptop) to USB-C (on the surface) cable to enable Inkback. "Allow input from a keyboard or mouse connected to this display." is checked on our laptop. Is there a step that we are missing? We still cannot markup the laptop image being projected to the Surface Hub 2.
Not sure which USB-C port on the Surface Hub 2S you are connecting to? For Touchback/Inkback you can only connect to USB-C on the back panel, one which is close to HDMI-in. (& not any of those guest USB-C ports on the sides / bezels of the Surface Hub 2S.
If you are already connecting on the back panel, try connecting the USB-A (laptop) to USB-C(Surface Hub 2S) first, then connect HDMI to HDMI.
Also, please check whether Touchback/ Inkback working thru miracast? (Connect feature on Windows 10)