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 need free space of about twice what 1SE already uses. Free up some space on your phone, or mash fewer snippets.
Here's a handful of tips from the 1SE community to reduce storage:
Mash annually and archive that project. A Pro subscription includes storage optimization and archiving.
You don't need to keep all your original videos from which you grabbed a second. Deleting them frees up a lot of space.
Be mindful of keeping a bunch of projects, such as Freestyle projects of trips or weddings. Mash those to have a keepsake, and archive them to save space.
On iPhone, turn on Offload Unused Apps in Settings > App Store to clear out old, bloated apps you've forgotten about.