Did you try it while in park or while underway. You may have to be in park.

 

I was in park. The option is there, but it is not selectable.

 

For me, it's grayed out when I first get in the car. Once I step on the brake and enter my PIN to drive (I have that enabled) slip start is selectable. I can enable it whether in park or drive. The manual says it's selectable at any speed, except when autopilot is enabled.