where are you browsing to? The Filepath has to be empty. This is not the "path" of your media files themselves.
for example the default "Main" Library is at C:\Users\S\AppData\Roaming\J River\Media Center 21\Library
you could put it in a created C:\Users\S\AppData\Roaming\J River\Media Center 21\ExtDrive Library for example or wherever you want. I find it better to keep it on the system SSD drive for speed though. But I don't think you have to.
I'd uncheck the Import the run import on empty libraries option. Configure your autoimport config, watched folders to the external drive path and then run a manual full import (autoimport).
You should be good to go with the new library containing just whats on the external drive. You probably should check that the library backups for your 2 libraries don't write to the same directory though
I'd recommend not tagging on the external drive though, and use the NAS files as the master (or vice-versa) if you ever want to sync them in the future. but that's for another thread