You can resolve this issue by reinstalling GarageBand.

1. Quit GarageBand.
2. In the finder, click the Go menu and choose Applications.
3. Drag the GarageBand icon to the trash.
4. Open the Mac App Store.
5. Click the Purchases tab.
6. Sign in to the Mac App Store using the Apple ID you first used to install GarageBand.
7. Look for GarageBand in the list of purchased applications. If you don't see it in the list, make sure it's not hidden.
8. Click Install to download and install GarageBand.