Troubleshooting & messages · Free space and clean-up messages
Why can't a local snapshot be removed because it's required by macOS?
Short answer: That snapshot belongs to macOS itself (for example, from a system update), and removing it could stop your Mac starting. Only Time Machine snapshots can be reclaimed.
Some snapshots belong to macOS itself — for example, the one your Mac boots from or one made during a system update. Removing those could stop your Mac starting, so macOS refuses and Storage Studio doesn't try.
They're usually small or temporary. Snapshots made for Time Machine are the ones you can reclaim.