If you’re trying to log in to the LINE app in China with a regular internet connection, then it’s just an exercise in futility. The app has been blocked in the country for years, and you’ll just keep getting an error message saying that the service is unavailable. The only way to overcome this barrier is by signing up for a VPN and changing your IP address to a country where the app is not blocked.
Follow the step-by-step guide earlier in this article, and the messaging app should work fine.