Users on the Professional and Professional Plus plans can add up to 8 songs to their video. If you have less than 8 songs in your video and you're getting an error that says, "Your Music Timeline is Full" you'll need to make space before you can add another one.
Occasionally, especially with longer videos you may have a song that gets trimmed down and is difficult to see. You can tell if there is free space because the timeline will have a grey background.
One way you can make space in your timeline is by shortening your existing songs. You can shorten your song tracks by clicking on the song you want to trim, and then moving the blue bumpers until there is space in the timeline.
If you want to remove a song, click on the track you want to delete. That will open the menu for that individual song. Click on the X icon to remove it from your project. Once you've made room, you're ready to add another song!
See also:
How to Add and Remove Additional Songs