Right, as Tom says, this is usually some kind of structural problem. But I think it can also be caused by running the training and creating a large memory footprint for your notebook. Here’s a cheap and easy thing to try first before you do the “Get a Clean Copy” procedure:
- Kernel → Restart and Clear Output
- Save
- Submit again
The point is that the grader does not need to see any of your generated output: it only needs to call your functions. And apparently having a large memory footprint for your notebook can mess over the grader. Here’s a thread where a student had exactly that error message and the “clear output” fixed it. And I think that was on this same assignment FWIW …