Check for any Samsung updates on your device.
Also, shake your device gently, and listen out for any rattling noises. There's a chance that your phone's camera sensor that detects the space between objects is broken, so it can't figure out the correct focus settings.
Additionally, you could always try putting your finger about 3 inches away from the phone camera and attempting to focus, then moving the finger further backwards until it loses focus (if it's too far, try using a cup on a table and walking back). Stop moving as soon as it loses focus, then measure the distance and cover each camera lens one by one until it looks covered on the camera app, this will tell you which camera is having issues (each camera works for a different distance). You can then try cleaning that one lens, or push comes to shove take it to your local phone repair store to fit a new lens.
