If you see Unable to Activate Tag while trying to activate your CGM device, it is possible that the tag is already active.
Try scanning the sensor by pressing the + button on the dashboard and selecting Scan from the menu. If the scan succeeds, then the sensor is already active.
If the scan was not successful, make sure when you are trying to activate the sensor that you are bringing the area of your phone where the NFC is located to the sensor. For iPhones, this is on the back of the phone right by the camera. Bring this area of the phone to the sensor and you should feel a vibration. For androids, the NFC location may vary. Make sure your NFC is turned on under settings. Bring the area of the phone where the NFC is to the sensor and hold for 1 second after you feel a vibration.
Screenshot of the error: