This link from the Now TV stick will show you all available apps for download.
Twitch appears to not be one of them
Hi @Anonymous User
Have a read of this Internet article on ways on watching / listening to Twitch.
Not familiar with Twitch and assume the Twitch Music App is the same has the Twitch App?
Unfortunately like @gavs82008 mentioned Twitch is not available via the NowTV branded devices.