While trying to install previous versions of Unity for asset compatibility purposes I ran into an error while using Unity Download Assistant downloaded from their page on Mac OS X Sierra 10.12.5. Here is the error:
After searching around I noticed that many people have this problem and nobody has a solution. After looking into it for some time it turns out the problem is in system permissions – it never asks for admin password before starting download and so it gets blocked by the system. To fix this run it as admin (you might need to change the path):
sudo /Applications/Unity\ Download\ Assistant.app/Contents/MacOS/Unity\ Download\ Assistant
File that is being ran as admin is inside the “Unity Download Assistant.app” package and can be seen by using right click > Show Package Contents.
Hope this helps somebody out there!