Apple - Unable to run xcode-select --install

I'd reboot the system and try again.

If that doesn't work, you can download it from Apple Developer.

If you don't have an Apple Developer Account, you can create a free account, and then once signed in you can download the "Command Line Tools (macOS 10.13) for Xcode 9.1" from: Downloads from Apple Developers

I had the same issue on macOS High Sierra. Not being a Mac enthusiast, I went online to find some answers, but found none.

I eventually figured it out (at least in my case, a MacBook Air running High Sierra). The window that states finding software actually had another window hidden behind it, with a prompt asking if I wanted to continue on battery power?

Installation happened right away afterwards.