Many SHSH errors are fixed by software updates. Open 3uTools, click Check Update at the bottom right, and install the newest version.
The "Unable to Request SHSH" error on 3utools can occur due to several reasons: How To Fix Error Unable To Request Shsh On 3utools When
to confirm if the firmware you are trying to install is still being signed by Apple. If there is a red "X" next to the version, you generally cannot flash it without previously saved SHSH blobs. Check Internet & Connection: Many SHSH errors are fixed by software updates