The error "Unable to Request SHSH" on 3utools can occur due to several reasons:

Check for the green "Signed" status next to firmware versions in the 3uTools "Easy Flash" section.

Leo sighed, leaning back in his swivel chair. SHSH blobs—the digital signatures Apple uses to verify firmware—were the gatekeepers of any successful downgrade or restore. Without them, the device was just a glass-and-aluminum paperweight.