Little Snitch by default ignores simulated keystrokes and mouse clicks to make sure that malicious software cannot change your firewall settings by simulating user interaction.
Unfortunately this also affects software that allows you to access your Mac remotely (like TeamViewer, RealVNC, etc).
If you do need remote access to Little Snitch, you have to enable the “Allow GUI Scripting access to Little Snitch” option in Little Snitch > Settings > Security.