The mod expects you to give a valid skin, and Melting by default does not have a C skin. Instead of typing what the skin will be (/autoskin Melting C rhino-skin), you should do /autoskin Melting A rhino-skin. This way it will generate the A skin sprites (you could replace A with B if you want B skin) in the %localappdata%/nuclearthrone/data/autoskin.mod directory within the folder name you gave (rhino-skin). After that you just edit the images and your skin should be ready for use.