I cannot install Rhino 6 Beta on iMac Mid June 2011 with macOS Sierra 10.12.6 installed.
Minimum requirement is High Sierra, but I think the recommendation is actually Mojave.
Also, some older AMD GPUs are not supported. @dan has a list of which they are.
I plan to update the system requirements on the website soon but, in the meantime, these are the GPUs that are NOT supported in Rhino 6 for Mac:
- AMD Radeon HD 6750M
- AMD Radeon HD 6770M
- AMD Radeon HD 6970M