Apple only "signs" (provides SHSH blobs for) the most recent iOS versions. Once a version is "unsigned," Apple's servers will no longer provide the SHSH blobs needed for a standard restore. Common Causes for the Error 3uTools - Facebook

The error message is a common hurdle for iPhone and iPad users attempting to restore or downgrade their devices using tools like 3uTools or iTunes . This error signifies that the software cannot retrieve the necessary digital signature from Apple's servers to authorize the firmware installation. Understanding SHSH Blobs

Requested firmware 14.8 for iPhone 12 is no longer signed.