macOS High Sierra 10.13 introduces a new security feature that requires user approval before installing new third-party drivers.
(Note - this doesn't affect drivers that were already installed before updating to High Sierra)
When installing True-Fi/Systemwide for the first time on macOS High Sierra, a pop-up similar to the screen shot below will appear. When you see a "System Extension Blocked" error message (screen shot below) please click OK. You will then need to ‘allow’ the driver install process to complete from the ‘security & privacy’ settings in system preferences” - please see the instructions set out in steps 1-4 below.
Please make the following changes to your Security & Privacy settings to enable the system extension:
1.) Open System Preferences, click on Security & Privacy.
2.) You will see a message: System software from developer "Sonarworks SIA" was blocked from loading.
3.) Click: Allow.
You may need to click the Lock symbol in the bottom right corner of your Security & Privacy menu to make this change.
4.) After clicking "Allow", restart the installer and it should now complete without errors.