If there's no clip or title placed on the timeline to match the music at the end of your project, this part won't be included in the downloaded file.
To make sure it's included, add a Title to the end of your project:
- On the timeline, place the playhead (yellow vertical line) at the point where your last clip ends.
- Tap on Title from the bottom tab bar: a blank clip will be added
Color: You can change its color or leave it black
Text: To remove the text, double-tap on the Text bar > tap Delete in the shortcut menu
- To change the length of the title, tap on its white side handle and drag right/left
You should now be able to save your entire project successfully.