Note: Startup disk isn't accessible on macOS Big Sur 11, Catalina 10.15, Mojave 10.14, or High Sierra 10.13 from any third-party applications due to System Integrity Protection (SIP). This feature prevents any modification in the protected system files and folders on your Mac.
To scan startup disk on macOS Big Sur or earlier using the latest version of the software, you need to allow Stellar Photo Recovery Extension from the software's interface. This extension is available in the Free, Standard, Professional, Premium, and Technician editions.
Steps to Recover Files from Macintosh HD:
Launch Stellar Photo Recovery for Mac on you MacBook, iMac, or Mac mini. Select Macintosh HD the startup disk, then click Scan.
In the message box, click Next. In the second message box, click Next again. In the System Extension Blocked dialog window, click Open Security Preferences.
In the Security & Privacy window, click the Lock icon, then provide admin password and click Unlock.
Click Allow to load the extension signed by Stellar Data Recovery. Click the Lock icon again and close the Security & Privacy window. In the message box, click Continue
Now, you can perform Mac Photo recovery on your startup disk by using Stellar Photo Recovery for Mac.
Note: If you've clicked OK on the System Extension Blocked message box, then you need to open the Security & Privacy window manually. Click the Apple icon and select System Preferences, then select Security & Privacy. Next, follow STEP 3 and STEP 4 as explained above.
Once the task is done, boot into recovery mode and type csrutil enable, and press Return. This will enable the SIP.