If you are attempting to downgrade and have previously saved SHSH blobs:
How To Fix Error Unable To Request SHSH On 3uTools If you are trying to flash or downgrade your iPhone, iPad, or iPod touch using and encounter the dreaded "Unable to Request SHSH" error, you are not alone. This error typically halts the flashing process at 9% or 10% and is often accompanied by "Unable to request SHSH" or "Apple is not currently signing this firmware" messages. How To Fix Error Unable To Request Shsh On 3utools When
Many SHSH errors are fixed by software updates. Open 3uTools, click Check Update at the bottom right, and install the newest version. If you are attempting to downgrade and have
Apple typically stops signing older iOS versions within a week or two after a new one is released, making them impossible to install through official means. Open 3uTools, click Check Update at the bottom
Use the to check if your desired iOS version is marked in green (signed) or red (unsigned).
A wrong ECID or device selection will cause an instant "Unable to request" error.