This error message can be caused by different issues:

1. The sensor ID is not correctly introduced in the doors configuration. The instructions to change it are here.
2. The sensor batteries ran out. The instructions to change them are here.
3. The sensor is not working, follow the instructions here to check if everything is correct.
4. 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.
5. In case there is an automation but you do not have an Apple TV or a bridge, it will not work.