If you are attempting to upgrade or restore to a currently signed version but the standard "Quick Flash Mode" gives you the SHSH error, try changing the flashing architecture.
Apple only allows the installation of "signed" iOS versions. If you are trying to downgrade or install an older version that Apple has stopped signing, the SHSH request will fail. You can check current signing status on sites like IPSW.me . i unable to request shsh 3utools fix install
Your computer’s firewall, antivirus, or host files are blocking 3uTools from communicating with Apple or 3uTools servers. If you are attempting to upgrade or restore
Before diving into the fixes, it helps to understand what is happening behind the scenes. You can check current signing status on sites like IPSW
Legacy versions of flashing tools suffer from expired network communication protocols, causing instant server rejections.
Sometimes, your local network or computer configuration misroutes the connection to Apple's servers.