Your ISP’s default DNS may cache old responses from Apple or block the request entirely. Switching to a public DNS resolves most "Unable to request" errors.
An outdated version of 3uTools might use obsolete protocols to request SHSH blobs. how to fix error unable to request shsh on 3utools when top
Before trying complex fixes, verify that Apple is actually signing the firmware you want to install. Your ISP’s default DNS may cache old responses
Example: tsschecker -d iPhone10,1 -e 0x1111111111111111 -i 15.7 1 -e 0x1111111111111111 -i 15.7