I cannot install Rhino 6 trial version on Mac OS 10.12.6.
Does anyone know how to make it work?
10.12.6 is not supported. The only way I know is to update your MacOS, assuming that is possible on your machine.
Is updating Mac OS is as painful as rewriting windows?
The times I have upgraded it has been pretty easy. New and changed features always have their drawbacks, but considering 10.13.6 is already pretty mature (and even that already nearing EOL in September this year…) I’d assume it is not too bumpy road. You have to make sure of course the hardware you have is supported by 10.13. Or newer if you think you’re up for making such a big leap.
Note that in 10.15 not all old software works anymore, so also verify your tools are supported by the verison you want/need to upgrade to.