To guarantee that the uploaded music is properly tagged and has metadata that provides high quality information about the songs, only files tagged with MusicBrainz metadata are accepted.
If your upload is failing, try to download MusicBrainz Picard: picard.musicbrainz.org
Use it to tag all the files you want to upload (remember to save them)
And then try the upload again.
Note: By clicking the ? button near the error message you can usually get more information about the error. If you read "Skipped" it usually means that the song is duplicated.
IMPORTANT: Make sure you upload music to a private library if it's under copyright, or it will be shared with everyone.