Troubleshooting & messages · Permissions and Full Disk Access
What does "Full Disk Access couldn't be confirmed" mean?
Short answer: Storage Studio couldn't tell whether it has Full Disk Access, so it safely assumes it doesn't. Switch it on (or off and on) in System Settings → Privacy & Security → Full Disk Access.
Storage Studio tried to check whether it has Full Disk Access and couldn't get a clear answer. Rather than assume it's allowed — and then fail halfway through a job — it assumes it isn't.
Open System Settings → Privacy & Security → Full Disk Access and make sure Storage Studio is switched on. If it already is, switch it off and back on. That removes the doubt.