GRUB is usually the last step of a Debian-based installation.. if you didn't get errors before, the system itself is probably installed, but the bootloader isn't for some reason. Unfortunately, I don't really know a concrete answer right now, I'm relatively sure it should be connected to UEFI somehow, but I can't tell you how to fix it. It has also usually "just worked" for me on any UEFI systems. You could once more try the same thing you did before: Boot your Ubuntu live system, click on "Try" and this time, enter the exact code from the Askubuntu answer I sent earlier, without changing anything. Copy the whole line into the terminal. Does that work? If not, I would try to go back into the BIOS and again disable everything that has to do with UEFI Boot / Quick Boot / Secure Boot and also set all the options to prefer or only allow "Legacy" boot, everywhere you find it. Then try another complete installation. If that doesn't work either, you might want to head over to Askubuntu yourself and open your own question or search for the error message text and try out the solutions that you find. This is technically not really an Ubuntu Studio issue, since it's related to the GRUB installation and your specific hardware, so you'll get better qualified people booting things all day on there, we're more concerned with the actual Studio software (Jack, DAWs, plugins, etc.) on this list. If everything fails, where did you get the PC from? If it's a vendor, they probably could troubleshoot/install the base system for you for a few extra bucks or maybe even for free, I know I would. But I'm also not a salesman (probably for that exact reason).