It depends on the sample rate of your device driver. I see you have a Studio 192 in your account.
When I run Studio One 3 Pro with Studio 192 connected and open a new Project, 176.4 is an option.
Make sure you have the latest driver of UC 2.0 installed which includes the latest firmware update for your S192.
If you're using another interface, their device or device driver may not support 176.4.
This screenshot is from Studio One Pro 3.3.4 build 41933 released Feb 17, 2017.