When the "Desktop & Documents" checkbox in iCloud Drive automatically unchecks itself after a few seconds, it typically points to an issue with local storage space, corrupted home directory aliases/folders, or an iCloud sync freeze.
1. Check Available iCloud and Mac Storage
- iCloud Storage: Ensure you have enough free space in your iCloud plan to accommodate the entire contents of your local Desktop and Documents folders.
- Mac Hard Drive Space: macOS requires temporary local breathing room to stage and migrate files. If your internal Mac SSD is nearly full, the system will abort the toggle. Go to Apple Menu () > System Settings > General > Storage to verify you have several gigabytes of free space.