This error message can be caused by different issues:
- The sensor ID is not correctly introduced in the doors configuration. The instructions to change it are here.
- The sensor batteries ran out. The instructions to change them are here.
- The sensor is not working, follow the instructions here to check if everything is correct.
- The ismartgate is not configured. The ismartgate is a Hub to link Homekit with a gate or garage door. To work it requires the device to be fully configured, at least one door enabled.
- In case there is an automation but you do not have an Apple TV or a bridge, it will not work.