First, make sure that you have enabled Locket to use the camera on your device. To make sure Locket can access your camera:
Go to device Settings
Go to Privacy & Security
Find and tap Camera
Find Locket, and make sure the toggle is green
If this doesn’t solve the issue, make sure that your camera is functioning well in the photo app first. If yes, try to exit out of the Locket app and enter back in.