According to a few commenters in the Apple Discussions support forums and a recent Apple knowledge base article, some may experience an issue using GarageBand '11 in which they are unable to create a new guitar track, even though they can create other real or software instrument tracks.
Apple says that the problem can occur if you chose to do a custom installation of a previous version of GarageBand and then moved the instruments and loops to a new location.
ASD forum user "Martyputz" writes:
I can create real instrument and software tracks, but NOT a new guitar track - nothing happens once I hit "create". UNLESS I start a new song with a guitar track, that works fine. But what if I want to add a guitar track after I start a song - no good. I can work around this by always starting with a guitar track and then duplicating that guitar track but this is a PITA.
Some commenters in the thread are blaming the GarageBand problem on the Mac App Store version of the software. Martyputz, later in the thread, writes:
Genius had to delete it and install iLife '11 from disc. They discounted it to the same price as the App store download, but in short - the App store has a defect here and others should be aware of it.
According to the KB article, the solution may actually be simpler. As it turns out, the GarageBand preference file holds the problem. Quit GarageBand, navigate to username > Library > Preferences, and find the "com.apple.garageband.plist" file. Drag the file to the Trash and restart GarageBand. You should now be able to create new guitar tracks in your projects.