Since macOS Mojave that introduced microphone permissions, all applications that use the microphone must be given permission to access your microphone, this applies to all input devices in general.
1. Navigate to System Preferences from the Apple Logo in the top left of the Menu Bar.
2. Select 'Security and Privacy'.
3. Select the 'Privacy' tab followed by 'Microphone'. Make sure that FL Studio is enabled (has a tick) to allow it access to your inputs devices for recording.
NOTE: If the FL Studio App does not show in the list on the right see here for a solution.
4. Click the lock icon to make/save changes (bottom of macOS Security & Privacy panel)
5. Restart FL Studio. FL Studio will now have access to your input devices for recording audio.
6. Ensure the Audio recording switch is enabled from the Main Record button (right-click) menu
For in depth information regarding Audio Recording in FL Studio refer to the manual HERE.
Image-Line Support Team