If your camera is working, it's likely that Locket doesn't have access to the camera. Follow these steps to ensure that Locket has permission to use the camera.
iPhone & iPad
Open Settings
Search for Locket
Make sure that the switch next to Camera is toggled on and green.
Android
Long press on the Locket icon from your Home Screen or App Drawer
Tap App Info
Tap Permissions
If Camera is in the Not allowed section, tap on it. Then select Allow.
If this doesn’t solve the issue, double check that the camera is working on your device's main camera app. You can also try restarting your device.