If you are trying to go back to an older iOS version that Apple has "unsigned," it is to fix this error unless you have previously saved your specific device's SHSH2 blobs. Without those saved files, there is no workaround to bypass the signature check.
: This is the most common fix. Older versions of 3uTools often lose compatibility with Apple's updated server protocols. Use the Check Update how to fix error unable to request shsh on 3utools when upd