First of all, our overlays are not available for macOS right now!
Make sure you are on the latest version.
You can check if an update is available under Settings > General > Application > Check for updates.
If not resolved Step 1:
If you have and use applications like Rivatuner and/or MSI Afterburner, disable or uninstall them. Unfortunately, these applications currently have conflicts with our overlay.
Ensure you have the overlay enabled. Move to Settings > League of Legends/Teamfight Tactics > Overlay and enable the respective overlay option.