The add-on is freely available on other platforms (Look at the links at the bottom of the About page of the addon). The $1 price tag for Blender Market gives you access to their download manager.

100% of the sales that normally go to the add-on creator (After Blender Market's share) is going to the Blender Development Fund, so I am not making any money out of the sales here. 

It looks like Blender does not have the access right to create a new folder within the addon directory.
You can solve that by opening blender with administrator rights.
You can do that by going in the properties of the Blender shortcut you use and enabling the "run as administrator" option, or by typing "blender" in windows search bar, then clicking on the right arrow of the blender shortcut, and then select "run as administrator".