Apple signs a new iOS version for only 1–3 weeks sometimes. The moment a new iOS drops, open 3uTools → Save SHSH → Save all available versions. Store these blobs on an external drive.
To understand the error, one must understand the request flow: 3utools Error Unable To Request Shsh
3uTools includes an "Alternative Mode" for requesting SHSH. This setting changes the way the software constructs the request sent to Apple’s servers. If the standard request fails due to server overload or specific device protocol issues, toggling this option in the "Pro Flash" settings often succeeds where the standard method fails. Apple signs a new iOS version for only 1–3 weeks sometimes