Good troubleshooting by trying that Tascam. It sounds like you know what you’re doing, but you HAVE selected your Focusrite interface within Cubase as your ASIO device? Do you have the proper drivers installed for the version of your Mac’s OS?
I can’t imagine that the Focusrite not compatible with Cubase, as far as I know it’s usually more of a computer hardware compatibility thing. I know nothing of Mac’s or Core Audio, but I’d ‘think’ that you should be able to open your audio interface without Cubase opened, so I’d be suspicious right there 