This is usually a result of a bad measurement. The system will automatically disable any assessments that it detects too much noise, incomplete data, or non uniform ECG signals. You should get a notification on this on the app.
Ensure that you have placed the sensor the correct orientation by checking that when sensor is snapped into the ECG belt that the cables and MicroUSB port are leading towards your waist and not your neck. If the port is facing your neck/head the ECG data will be invalid.