In the transport module you can see to which bit depth your project is set.
You cannot change the bit-depth of a recording in a project. When a project is set to 16-Bit you cannot record with 24-bit by changing anything in the window of the DuoCapture.
At the beginning of an empty project you are asked for the bit-depth. Choose 24-Bit. Only than you can record at 24-Bit.
Also in preferences -> file -> audiodata you have to choose for recording bit depth "24 Bit".
If you want to change an existing project from 16 to 24 Bit than go to Utilities -> Change Audio Format. But do also the changes in the preferences.