Then reconnect your iDevice, let 3uTools re-detect it, and retry the SHSH request.
Use the feature in 3uTools to ensure your connection drivers are functioning correctly. 3utools error unable to request shsh hot
Before flashing, ensure the iOS version you want to install is still being signed by Apple. 3uTools Error: "Unable to Request SHSH Hot" –
Temporary downtime on Apple's side or a firewall/connection issue on your PC. Solutions & Troubleshooting let 3uTools re-detect it