For registration method, see here.    

1

Out of all smartwatches, only Apple Watch can be registered to FamiLock.
For applicable Apple Watch, see here.    

2

There is a possibility that Bluetooth might be turned OFF.
Please to Settings screen of the Apple Watch and make sure Bluetooth is ON.

3

The device might have been registered in the past and Bluetooth information remains: 
Go to Settings, then Bluetooth. If 'FamiLock' appears in the Devices list, tap the info (i) icon and select 'Forget This Device'. If multiple digital locks are registered, 'FamiLock' will be listed for each lock. 
Due to watchOS specifications, it is not possible to distinguish which entry corresponds to which lock. 
If incorrect lock is unpaired, you will need to register that digital lock again.

4

If no digital lock was detected, 
the cause is likely to be the following.
・No smartphone was registered. 
   Please register at least 1 smartphone.
・No digital lock within range.
 Please perform registration near a digital lock.
・Registering Apple Watch was once used to registered in the past, and its data still remains in the digital lock.
   Please use smartphone to delete registration data of that Apple Watch.

To delete Apple Watch registration data, please see here.

5

Digital lock might have reached its registration data limit.

・BLE device has reached its data limit of 10 registrations.
Please delete digital lock registration data by using your smartphone.

For how to delete Apple Watch registration data, see here.

6

Apple Watch might have reached its registration data limit.

Please delete digital lock registration data from your Apple Watch.

For how to delete Apple Watch registration data, see here.

7

The Apple Watch might have been registered to the digital lock. 
Please delete digital lock registration data from the Apple Watch.

For how to delete Apple Watch registration data, see here.
For how to delete digital lock registration data from Apple Watch, see here.