The Mac has a forgotten firmware lock. Fix: You must take the Mac to an Apple Store (with proof of purchase) or use a known firmware password reset tool for legacy Macs.

: This method only works if your Mac model was originally compatible with High Sierra. Alternative: Using the Mist Utility

Do not search for raw DMG files. Step 2: On any working Mac, run:

Systems Analysis Team Attachments: None – distribution prohibited per Apple license.

bypass "This copy of the install macOS application is damaged" errors that sometimes occur with older installers?

Ensure you have at least 15–20 GB of free disk space for the installer and the installation process. 3. Install macOS High Sierra

Copying installer files to disk...