swift run docc preview TSPL.docc means you will call swift compile to build and run a package's product named "docc" and pass "preview TSPL.docc" as its arguments params so it needs a file named Package.swift in the current working directory.
However according to README.md of swift-book, there is no Package.swift and you should call docc preview TSPL.docc to directly preview it. If you do not have docc in you $PATH, you can use xcrun docc to find the docc executable of your Xcode's default Swift Toolchain on macOS.
so the final command you may consider to use is. xcrun docc preview TSPL.docc