If you’ve just upgraded to macOS 10.13 High Sierra from 10.12 and are wondering why Safari Technology Preview is crossed out with a white 🚫 prohibited/cross mark and unable to be launched, turns out there is a separate Safari Tech Preview version for High Sierra. Confusingly, the App Store / updater does not automatically upgrade to this version when you install High Sierra, so the old version left over in /Applications is just…sitting there, broken.
Go back to the Apple Developer downloads page for Safari, grab the “Safari Technology Preview for macOS High Sierra” installer, and install it over the existing Safari Technology Preview. The app should then launch.