Canon 6D is not detected by Capture One.
Canon 6D has two sharing modes: Wi-Fi and USB. If you tether with the help of USB, it will not be possible to have both sharing modes activated. Disable Wi-Fi and tethered ability will be restored (you may also need to unplug and re-attach the camera for Capture One to detect it after switching off Wi-Fi).
This issue was noted in Canon firmware 1.1.3, however, it may occur in other firmware versions as well.