3uTools requires the iPhone to be in a predictable state. If your device is in a boot loop, in DFU mode with corrupted drivers, or simply not authenticated properly via USB, the SHSH request will fail.
3uTools is a third-party Windows/Mac utility. It tries to request these signatures from Apple. When it fails, you see the error: "Unable to request SHSH."
to manually retrieve or check for saved SHSH blobs. You may need to copy your device's to use these services. Update iTunes and Drivers