When setting up Family Link for my kid, the setup keep failing and just showing very unhelpful error message "Something went wrong ..". Since the phone doesn't has family link installed yet, I try to login with my account first and installed Family Link app.
Opening the app, on my kid's account status, I can see it shows "Device isn't ready for ...". I removed my account (since family link account can't share device) and try to add my kid's account again. This time the setup went through!
So if you're facing similar issue and doesn't has Family Link app installed on the device yet, try to install the app with your own account first, before setting up your kid's account.