If 3uTools continues to fail, you can try to manually retrieve and import your SHSH blobs:
: Apple stops "signing" older iOS versions shortly after new releases. If the version you are trying to flash is no longer signed, 3uTools cannot request the necessary SHSH blobs from Apple's servers. Check Network & Connections : i unable to request shsh 3utools fix better
Users on community forums have shared specific workarounds that helped them when the standard "Easy Flash" failed. If 3uTools continues to fail, you can try
Switch from a Wi-Fi connection to a stable USB tether or Ethernet. Switch from a Wi-Fi connection to a stable
Have a specific error code from 3uTools? Share it in the comments below. For signed versions, the fixes above will work 99% of the time. For unsigned versions, let go of the hope—no tool can change Apple's cryptographic signature policy.