You asked this question slightly differently elsewhere, and I answered it:
You store music on the internal Music folder either by:
- copying music folders over the network to the InternalStorage folder, or
- Dragging and dropping the folders onto the Roon window, and Roon will import them into your library, or
- If you have a CD drive attached to your NUC, you can rip CDs using it, and Roon will import the ripped files into your library.