iMovie for iOS - can't export due to storage issue

I've made a video, 20 minutes long, on iMovie, but it is not letting me export it as it says there is not enough storage space. When I have checked "settings - general - iphone storage", there is plenty of free space (it says 74.9gb used out of 128gb), so I don't know why this is happening. I don't want to lose the project by deleting the app and reinstalling but I need to be able to save it elsewhere rather than just on the app. Any ideas please??