add music to an ios device from itunes 12 running windows 10

I have to assume then that the "add to device" option was because you were using Manually Manage (which I've never used). However, you mentioned Sync. For a Sync to add music to your device, you need to set Sync options.

Connect the iPod to iTunes and use the iPod icon on iTunes' menu bar...

... to look at the Settings/Music pane (1.):

• Is your Sync Music option (2.) ticked (selected)?
• If it is, which option in (3.) do you have selected? Note, if you have Entire Music Library selected, you will not see the boxes numbered (4.)

How are you attempting to add the music? Are you using Sync, or Manually Manage, do you have iCloud, do the songs play in your Tunes Library?

If you outline the steps you're taking and then tell us at which point it doesn't go to plan, perhaps we can help.

I don't recognise your method at all.

Have you added music to this device from another iTunes Library, that is still on the device now?

But have you added music to this device from another iTunes Library, that is still on the device now?