So the easiest fix would be to clean up your system with CleanMyMac X before upgrading. If you install Catalina on top of your current operating system, the computer will keep all the files and still need free space for Catalina. Then step 4 is exit utility, click on macos install, and install to mymacos volume. My step 3 is to use the disk utility to erase/partition the second volume (mymacos) to APFS, leaving the bootable volume containing the new 8 gb OS install file untouched (in my case I used Catalina).
Can't install HS onto that disk, guessing because of an EFI check happening on that disk. The scrap 840 EVO Drive I was going to install High Sierra onto also has a separate High Sierra partition from my real Mac Pro.
