The cause is one of the followings.

1

Bluetooth function might be turned OFF. 

Please turn ON Apple Watch Bluetooth function.

2

Apple Watch might be outside of transmission area.

The operation distance might vary upon the nearby surroundings and obstacles.

Please change the location and try again.

【Caution】
Operation by application is based on the usage distance that the eye can see. (within 10 meters) However, please keep in mind that the area can be extended further due to the characteristic of Bluetooth. 

3

Apple Watch registration data might have been deleted via smartphone app. 
Please consult with your family members if they have deleted your registration data and register your Apple Watch again. 

4

The door unit may be experiencing an internal transmission error. 
Press and hold the registration button on the door to enter registration mode. 
Once registration mode has ended, please try operating the app again.

5

There might be an abnormality in door's power source.

【When using batteries】
Please reinsert batteries in correct manner or change the batteries.
How to change batteries

【When using AC adapter】
Please make sure that the AC adapter is connected with the power outlet.

【When using controller unit】
Please make sure that controller unit's power is turned ON.