When it says missing files, it means somethings in the wrong place, and usually its object textures. However, I believe your problem is your custom cartexture.
The track packager hasnt been updated to include double cartextures as of yet I dont think, so its not clever enough to see that you have two textures instead of the usual one. It should still be fine, but you'll need to upload the custom textures yourself elsewhere, preferably in a zip file so its easy for people to download.
If its not that, then take a look at what it says when it says missing files to see what the filename is.
Hope this helped! =)