Since iOS 12.1 is blocked, you should install the latest supported version for the , which is . Over-the-Air (Easiest) Open Settings > General . Tap Software Update . Select Download and Install . Using a Computer (Manual IPSW)
You cannot downgrade from a newer version (like iOS 16) to 12.1. 📥 How to Download (Archive Only) Download iOS 12.1 for iPhone 8 (GSM)
If you have a signed IPSW file and want to install it manually: iPhone 8 (GSM) IPSW Download - iClarified Since iOS 12
iTunes or Finder will block the installation of unsigned firmware. Since iOS 12.1 is blocked