If you have a signed IPSW file and want to install it manually: iPhone 8 (GSM) IPSW Download - iClarified
iTunes or Finder will block the installation of unsigned firmware. Download iOS 12.1 for iPhone 8 (GSM)
Apple uses a "signing" system to control which iOS versions can be installed. iOS 12.1 is currently unsigned. If you have a signed IPSW file and