I had some time to kill, tried this now, and can’t duplicate. I downloaded the latest release build, scp’d it to the Mac, plugged in my keyboard, unzippped, and ran open Onslaught.app from a terminal. Everything ran and spoke. I confirmed that I’m running the latest build by switching focus modes, which is a new feature.
Not sure what else to do for you. I’m on MacOS 10.15.6 and a newly-purchased Mac Mini. Given that I only use this for development, I haven’t changed its settings much. Are you using the default voice? I don’t know if that makes a difference, but if so then that’s a bug.
Given that you’re an iOS developer, any help you can offer to debug this would be appreciated. Otherwise, I’m not sure what else to do for you. Sorry. :(