If your goal is to downgrade to an older, unsigned version of iOS, you will always encounter the "Unable to Request SHSH" error. This is because Apple's servers will no longer provide the ticket for that version.
For downgrades to unsigned firmware using blobs, your device must be in "Pwned DFU" mode (a special state that disables signature checks). how to fix error unable to request shsh on 3utools when upd