This is because your data contains only 4 peaks. Mummuchog expects the complete peaks list from high-resolution LC-MS (usually >1000s peaks). You can see more discussions in this post
I also tested our example data, it works as expected. As there could be many ways that cause an error despite the best intention, please follow our post guideline if you believe the issue is related to our example data or tools (not your data or operations)