Check the Load terminal (red screw) has a wire properly attached (typically black). If the load is on or flickering and there is no wire attached to the red screw, the device is not properly wired. Most likely the black is attached to BK black terminal and the white was attached to WH white with nothing attached to the red. If that is the case, see the video How to Identify a Neutral Wire to determine if you have a neutral in the box. Decora Smart devices require a neutral wire. If you are unsure if you have a neutral, please contact an electrician.
Link to How to Identify a Neutral Wire Video: