It seems like 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 smaller compilation for it to work.
We're working on improving our compression a bit, creating a backup system, and refining how we store files. In the meantime, here's a handful of tips from the 1SE community to reduce storage:
Mash annually and delete that project. We recommend backing up your 1SE before doing this just in case. Our Pro membership offers unlimited backup of everything!
You don't need to keep all your original videos you grabbed a second from. Delete those bad boys and a bunch of space will free up!
Be mindful of keeping a bunch of projects. Like Freestyles of trips or weddings. Mash those to have the keepsake and delete the projects to save space.
Consider using Apple's clean-up unused apps options in your settings to remove old bloated apps you've forgotten about
Hope those help a bit!