Check that the image used inside of the project’s content folder and not an external / network drive. If the decal is spawned in or streamed in then you may need to add the asset in to manually cooked items.
If your assets are not cooking then maybe the redirector paths are not fixed in the project and the engine is not cooking them properly. You can try calling fixup redirectors from the content browsers context menu and recooking.
You can also add specific paths for assets that are spawned at runtime.
Run cooking content manually and check the output to see if there are any errors.