Chromecast not available on Windows Desktop?

[diskowitchdocto]

I'm unable to cast from my Windows Desktop App. Details,

1. 'Cast' option is grayed out in the App (screenshot attached).
2. Chromecast is on the same network as my laptop with the App.
3. I have a Premium Family subscription. 

My understanding is that this option should be available to me if (2) and (3) are satisfied, I should be able to cast from the Desktop App. Is my understanding incorrect? If so, does anyone have any suggestions for a fix?