If Apple is not signing the iOS version, you cannot magically request a signature through 3uTools or any other tool. The only exceptions are:
Sometimes, 3uTools or a jailbreak tool modifies your computer’s to redirect SHSH requests to Cydia’s old server or a local TSS server. This causes the “Unable to request” error because Apple’s real server is being bypassed. How To Fix Error Unable To Request Shsh On 3utools When