darwinup: command not found

Building Darwin from source is quite different now that Apple has discontinued that tool.

The technical problem is that the darwinup utility is not compatible with the security requirements of Big Sur and later macOS versions, so it's longer distributed as part of macOS — and it's not safe to use on current macOS versions even if you have the utility.

• Where is darwinup to install SimpleAudioDriver in command line script?

Unfortunately, unless you can work on older versions of macOS, engineering your tool chain to use other tools may be your next move. It’s hard to know, though without some context to your end goal.