The video editor builds your MP4 inside your browser, which needs a recent version of Chrome, Edge or Safari.
You may also see:
"This browser cannot export video. Try the latest Chrome, Edge or Safari."
"Add at least one scene first."
What to do
Update your browser, or open Studio in the latest Chrome, Edge or Safari. Your video is saved, so it opens just as you left it.
On iPhone and iPad, use Safari.
Add at least one image or clip with Add scene before exporting.