I’m having a similar issue running Cubase 6, so I’ll tell you where I’ve got to in the hope it works better for you (or gives you an idea where to start).
Cubase 6 ran on Yosemite perfectly well on my iMac which I’ve just replaced with a Macbook Pro. As the iMac was running OS X Yosemite I assumed it would be OK on the MBP. It wasn’t, so I attempted to reinstall (at which point Cubase told me I was running an out of date version of OS X and wouldn’t install)
I eventually managed to reinstall Cubase using the Yosemite install tool (see the link): https://www.steinberg.net/en/support/content_and_accessories/content_and_accessories_saitfy.html
I also had to update the eLicenser software (I migrated from the iMac so lost certain settings): https://www.steinberg.net/en/support/knowledgebase_new/show_details/kb_show/usb-elicensersteinberg-key-details-tips-and-troubleshooting/kb_back/2020.html
I can launch Cubase 6 now and open projects but it doesn’t work properly (no sound and the mouse pointer doesn’t line up with the UI properly)
I’m still looking for more options to get Cubase running like it was on my old computer, so this is probably only step one for you too, but hope it helps in some way…