You can only revert to the firmware version that was installed on that specific console immediately before the current update. If your console is on firmware 11.50, and it was previously on 11.02, you can only go back to 11.02. You cannot skip backward multiple versions to reach a jailbreakable firmware like 9.00 unless that was your immediate last update.
Perform a clean install of the target firmware via safe mode. downgrade tool ps4