It's trying to run `WHISPER_EXECUTABLE` which it defaults to `whisper` - but if you follow the instructions in the README, you end up with `main`.

    WHISPER_EXECUTABLE=main examples/yt-wsp.sh youtube_url

Or the script itself prints out a handy hint.

    Whisper needs to be built into the main binary with make, then you can rename it to something like 'whisper' and add it to your PATH for convenience.