Your network signal may be too weak, or the device is added by another person.
Solution: Reset your device, and try adding it again. Make sure your phone’s Bluetooth is on during the whole process.
It may be because the device to be added is not available in this region. Please ensure that the device is available in the current region. Please ensure that the setting in Profile > Settings > Region on the Mi Home or Xiaomi Home app is the same as the region the device was purchased in.