I have been able to use my external interface with no issues but when I restarted without my interface, opened cubase and was unable to switch the output to my built in outputs. It is an option but when selected it goes right back to no outputs.
Mac - High Sierra 10.13.4
I think that Cubase has issues sometimes with ASIO drivers for devices that are not on at the time Cubase loads. It’s basically a setup failure. I’ve always found that if you quit Cubase, then restart Cubase with the device(s) on, then the problem goes away.
In studio > audio connections you have the option of saving your setup as a preset on each tab. I’d recommend doing that when you have everything working. That way, if Cubase somehow messes with those settings due to this problem, you can recover your known, good configuration. It also seems like a good idea if you need to switch up your audio connections for any reason. (For example, using a different audio interface).