Sometimes some of the assets (textures included) get damaged for various reasons: artist errors, broken merges, file system or software failures. Whenever Luna attempts to export a texture and is unable to get its pixel data, it will report this message to Luna expert.
Not really. Luna is using multiple techniques to gather texture data, starting by making a copy of the underlying texture source file in case it is PNG, up to utilizing Unity's Graphics API to copy pixel data to work around compressed formats. Thus, if Luna fails to extract pixel data from a texture, it is most likely damaged in one way or another.
Medium / High
Fix the underlying asset.