This could happen because the Smart AC Control V3+ is not positioned correctly. Or because the Smart AC Control V3+ is not sending the right commands to your AC.
Make sure that the Smart AC Control V3+ is in the direct line of sight of your AC unit. And that it is not too far apart from the AC.
If your Smart AC Control V3+ is positioned correctly, then go to the respective room settings in Settings > Rooms. Try resetting the installation process by choosing Restart AC Setup.
In the non-thermostatic operating mode, if your AC reacts to the ON command on tado° (i.e. it beeps), but it does not switch on, please contact our Technical Support.