This message is caused by material that is not supported by Corona being present in the scene. Corona will render such material with red warning-labeled material.
Possible causes:
The scene contains one or more materials that are not supported by Corona. This often happens if the objects with Corona-incompatible materials are loaded into the scene from an external source (for example by merging scenes). See this list for all materials that Corona supports.
Solutions:
Remove offending material from your scene and replace it with Corona-compatible material - this can be easily automated using the Converter Script.
Unsupported material displayed in rendered image