He’d seen this before, but tonight it felt like a wall he couldn't climb. He knew what SHSH blobs were—those digital signatures Apple uses to decide which iOS versions are allowed on their devices. If Apple isn't "signing" a version anymore, you can't install it.
Switch to an original Apple USB cable and try a different USB port (preferably on the back of a desktop PC). Check Network: 3utools Error Unable To Request Shsh