If the server is genuinely not signing the firmware, but you have previously saved your SHSH blobs (via TSS Saver), you can use them locally. Ensure you have your .shsh2 file.
Corrupt drivers on Windows are a frequent cause of communication errors between 3uTools and iOS devices. how to fix error unable to request shsh on 3utools when top
The most common cause is a network restriction blocking the connection to Apple's servers. If the server is genuinely not signing the