Hi and welcome to the forum,
Cubase 11 is not officially compatible with macOS 10.13 High Sierra. Crash of Cubase is a known issue on this system. Here you can find the main thread about the crash.
Cubase 11.0.30 isn’t officially compatible with macOS 10.13 neither, but it seems, it works. Delete Cubase 11 from Applications folder. Install Cubase 11.0.30 from your Cubase 11.0.30 installer backup, or download the installer here, please.