On A12+ devices (iPhone XS and newer), the ApNonce changes randomly. 3uTools sometimes fails to request SHSH because it cannot lock the nonce. You must generate a fresh one.

These often block the specific ports 3uTools needs to talk to Apple. Repair Drivers: in 3uTools and click on iTunes Utility Repair Driver

You’ve got your firmware ready, your device is connected, and you’re set to flash—only for 3uTools to stall at 9% with the frustrating message: "Unable to request SHSH."

Use official cable + direct USB port