This is almost always a device permissions issue. The FlipperForce Field app needs access to your camera and photo library, and those permissions are granted the first time you use the feature. If you declined at that prompt, the app cannot open your camera or your photo library.
Open Settings, scroll to FlipperForce, and turn on Camera. For Photos, select Full Access or Limited Access. If you choose Limited, you will only be able to pick from the photos you have specifically allowed.
Open Settings, then Apps, then FlipperForce, then Permissions. Set Camera and Photos and videos to Allow.
Close the app completely and reopen it, since permission changes sometimes need a restart to take effect. If the problem continues, send us a chat message with your device model and OS version.