COME ON I JUST WANT TO IMPORT MUSIC!

https://piskel-imgstore-b.appspot.com/img/e83ce911-1287-11e9-91b8-8f2805635641.gif

Arg, yeah... the import music is quite sensitive... (Maybe your song's names have spaces or special characters?)
I would suggest you to check the Music section of the forum, some interesting posts about limitations of the music for NESMakers, what you can't do, what you need to check...
And this post: http://nesmakers.com/viewtopic.php?p=8897#p8897 and http://nesmakers.com/viewtopic.php?f=4&t=219

I give up with this program, it is a buggy mess that can barely do anything. You are better off learning actual 6502 Assembly and coding NES games yourself, don't bother with this.

Which version are you using?
If it's the 4.1, I will suggest you to follow ALL the new tutorial videos from www.thenew8bitheroes.com website. It helps a lot!

I got that same error. Make sure that if you have any placeholder songs that everything has at least 1 note recorded.

The error means (I think) that something you are trying to import is empty.

I have blanks set up for all the songs I know I want, but some of them have nothing in them.
If you want the whole song set to import correctly just put a garbage note as the first note of the song until you get to it.

The 4.1 tutorial helps a lot. I was too overwhelmed to even try music until I saw the video.