Hello,
You’re getting the OS version error because the installer only checks the first digit after the decimal and thinks you’re running OS10.1.
When Nu7 had issues with large track count recording on the Mac we had all 9 of our systems running Nu5.5 on OS10.10 by modifying the sysver plist during the install process. I recommend that no one do that! But it does work if you time it correctly and aren’t afraid of Terminal, and got us through that painful period until Steinberg figured it out.
You’re liable to have issues with APIs that Nu5.5 uses and Apple has since deprecated between then and OS10.13. I don’t know which but Apple frequently deprecates graphics routines, network calls, ownership calls, etc. So it may work and it may work only partially.
Good luck.
Hugh