If you have tried all the steps above and are still getting "Unable to request SHSH," it might simply be a waiting game. The developers of 3uTools usually push a hotfix within 24 to 48 hours of a major iOS release.
: For some specific device models, you may need to copy your device's EID code and verify the firmware availability on third-party verification sites like SH.host to ensure the version you have matches the expected signature.
tsschecker -d iPhone15,4 -e 0xYOURECID -i 18.0 --save-path ./blobs
Manually put your iPhone or iPad into (the steps vary by model).