Beta Demo for Death's Gambit is a special access feature given only to select streamers. Regular players cannot access the demo, and must wait for official launch to play the game.
Beta Demo for Death's Gambit
How to access the Beta Demo
Only those with a streaming or broadcasting audience that the developers want to reach may get access to the demo. If you believe you cover the requirements, get in touch with them via twitter, discord, or their official website.
From the official FAQ:
"The demo is currently only given out to hand selected broadcasters. You can't get the demo.
But what if I keep asking nicely? Thank you for your kindness and decorum, but no, no demo. Sorry."