This message means that some of the materials in the rendered scene are not compatible with Corona. This usually happens when using materials native to other renderers.
Please either manually replace the problematic material with a Corona-compatible one, or use the Converter Script to automate the process for large scenes with high numbers of materials.
Note: sometimes this message might be displayed when using the 3ds Max Hair and Fur modifier. This means that "mr prim" rendering mode is set in Hair and Fur options in the 3ds Max effects window (8 key) - please make sure that either "geometry" or "buffer" is selected.
Unsupported material displayed in a rendered image.
Unsupported material displayed in the material editor.