With the release of macOS 10.14 (Mojave), Apple has released a new security feature which requires the user to manually "Allow" apps to access the microphone. When opening an audio app for the first time you will be presented with the below message.
In order to input audio into your apps using your Focusrite interface, you will be required to press "OK".
If you have clicked "Don't Allow" for any reason, you will need to open the System Preferences and go to Security and Privacy to manually allow the app to access audio input. Once you're in Security and Privacy, click on the Privacy tab, then select Microphone on the left-hand side and select the apps you wish to access the microphone.