The full message ends: "Add scenes, or the end is cut."
Each scene can last up to 120 seconds, and a video clip cannot run longer than its source. Your scenes together are shorter than the voice-over, so the end of the voice-over would be cut.
What to do
Tap Add scene above the scenes to add more images or clips. Then open the Audio tab and tap Fit scenes to it.
Or shorten the script: in the Audio tab, tap Generate new, edit the Script and tap Generate voice-over.