We've been tracking an issue with realMyst: Masterpiece Edition crashing at startup on all Macs running any versions of High Sierra.
We're in contact with our realMYST developers and working on a solution to this problem.
We recommend for the time being that, if possible, you get a refund from the retailer you purchased the game from, providing them the details of why you're requesting the refund, and that Cyan is aware of the issue and taking steps to find its source and correct it.
We do not have any ETA on when the fix will be implemented. Thank you for your patience as we work to correct the problem.
We're sorry this happened, and hope once the game is fixed you may try it once more. If you happen to have a Mac that is still running 10.12.6, we did check that OS and it appears to run realMYST: ME just fine.