hendrik on 12 Sep, 2017 07:32 AM
MusicBrainz is a website that aims at providing correct metadata for all music. It's created and edited by volunteers. While millions of songs are in there and therefore have ids, plenty of songs don't. In essence, a MusicBrainz id is the last part of a URL like https://musicbrainz.org/recording/e57c0b01-110e-4ea1-9dca-6e3c9ac33ca4, in this case pointing to the recording "Sekt zum Frühstück". Its id is e57c0b01-110e-4ea1-9dca-6e3c9ac33ca4.
MusicBrainz serves many other services as a reference. Therefore, MusicBrainz ids are often used to identify a given title. E.g. Last.FM uses MusicBrainz ids in their API, as well as AcousticBrainz.
In order to look up data from third party services like AcousticBrainz, beaTunes needs to first figure out the MusicBrainz id for a given song. Then, using that id as a handle, it can look up data from such other services. beaTunes uses its own database to look up the MusicBrainz id. When you're seeing the message, that beaTunes was unable to find a MusicBrainz id, it simply means that either the song does not have a MusicBrainz id yet or that beaTunes could not find it in its own database.
If you're interested in finding out what the cause of the problem is, you can go to MusicBrainz and look up your song manually. If you cannot find it, it's simply not in the database yet. You might want to add it.
If it's on MusicBrainz, but beaTunes still cannot find it, either it's not in the beaTunes database or it cannot be found (due to a different spelling or the like). In that case, you might want to use the Picard tagger to add ids to your files.