If you see this error message when exporting your movie to your phone:
Make sure you have enough storage space for the app to make a compilation. It would help if you had at least the double of the total outcome of free space. For example, if 1SE is taking 2GB on your device, you need to have 4GB of storage free on your device. Here are some tips to make the app take up less space on your phone.
If you have enough storage space, a corrupted snippet is likely causing this issue. Please try mashing a different set of dates, or creating a smaller compilation. You can experiment with different dates until you find the snippet, in particular, that's causing the error. Then, delete this snippet, "reset" it using iMovie, and re-add it to the app.