iCloud & iCloud Photos · Troubleshooting iCloud space
Why doesn't iCloud Drive appear in Storage Studio's Optimize Cloud Storage menu?
Short answer: Storage Studio only lists cloud folders it finds on your Mac. Turn on iCloud Drive in System Settings → your name → iCloud, let it finish setting up, then reopen the menu so Storage Studio re-scans.
Storage Studio lists the cloud folders it finds on your Mac, so iCloud Drive only appears once it's turned on. Check System Settings → your name → iCloud and make sure iCloud Drive is on and has finished setting up.
Then refresh the list: reopen the menu (it re-scans), or use Refresh in the submenu where it's offered. If you've just signed in to iCloud, give it a minute to create the iCloud Drive folder.
If you sign out of a provider, its row disappears the next time the menu opens — that's expected. If iCloud Drive is on and still doesn't appear, make sure Storage Studio has Full Disk Access, then quit and reopen it.