Your phone doesn't have enough space to finish the compilation.
If you're trying to create a mash with all of the snippets in your app, you will need about twice the size of the space 1SE uses in your disk to put it all together.
Please free some space on your phone or create a more petite compilation for it to work.
Here's a handful of tips from the 1SE community to reduce storage:
Mash annually and archive that project. Our Pro membership offers storage optimization and archiving.
You don't need to keep all your original videos from which you grabbed a second. Delete those, and you'll free up a lot of space!
Be mindful of keeping a bunch of projects, such as freestyles of trips or weddings. Mash those to have a keepsake, and archive them to save space.
You should consider using Apple's clean-up unused apps options in your settings to remove old, bloated apps you've forgotten about.