This error typically triggers when the —the software that tells the hardware how to start up—cannot be located or verified by the flashing software. This happens for three main reasons:
Ensure you aren't using a "Global" firmware for a "GSM" device (or vice versa), though most modern iPhones use unified files. Step 2: Swap Your Connection Hardware
Visit IPSW.me and ensure the firmware version you are using has a green checkmark. Apple stops "signing" older versions shortly after a new update. If it's not signed, the loader will fail every time.
The Apple servers are not signing the specific firmware version you are trying to install.