If you're getting an Access Denied error message when launching Reason 12 on your Mac, here are some things to check:
Open macOS "Security & Privacy" system preference and verify that that Reason 12 is allowed access the Downloads and Documents folders, in Privacy/Files and Folders.
If Reason is allowed access to the Documents and Downloads folder and you're still unable to launch, try deleting the "Music/Propellerhead Content" folder and see if it helps.
You can also try to wipe Reason's VST settings, to make Reason rescan your default plugin folders.
Info about all currently installed VST dll's are saved by Reason in a file called "__PluginDatabase.dat", "__PluginDatabasev2.dat" and "__PluginDatabasev3.dat", and which folders to scan is stored in "Reason Plugin Dirs".
You'll find the "Reason Plugin Dirs.prf" file in the "Preferences" folder in the user Library folder.
You'll find the "__PluginDatabase.dat", "__PluginDatabasev2.dat" and "__PluginDatabasev3.dat" files in the "Application Support/Propellerhead Software/Reason/Caches" folder in the user Library folder.
The user Library folder is hidden by default. To find it, press Option key while selecting Go from the menu in Finder, and then select Library.
Delete the preference files mentioned above, then launch Reason. This will force Reason to completely rescan the default VST folders.
If this works, add any additional VST folder(s) you might have on the Folders tab in Reason's Preferences. If you add an additional VST folder, remember to restart Reason to scan the new folder.