Hover over the folder in the Reuse Library that you are right clicking on to get a tooltip showing the path to that folder. Make sure you have write access to that folder. If NX was installed as administrator, you might need to run NX as administrator in order to write to that location.
Alternatively, use Library Management to add a new folder to a writable location, or edit the Customer Defaults to add those writable locations. In my case, NX is installed on a network location that I don't have write access to, so those options are greyed out for me too for the default Reuse folders, unless I use locally installed versions of NX (where NX was not installed as administrator).
Yep, Windows Security was locking me out. My account is a local Admin so I'm still not sure why I was being locked out. I did change the settings for "Everyone" to give them Full Control over these folders.