The difference would be that he happened to have a row highlighted, while you apparently did not. It’s a subtle bug that bites many of us. One of those things that make you wonder, “What were they thinking?” Having similar properties with similar functions, and only one character different? It’s a gotcha. Enough so that I recognized it right away, I’ve been bitten so many times.
Based on that tutorial, you need to be using ListIndex, not LastIndex. The code is specifically to put an “X” on the row that is selected in the ListBox. If you click the “Complete” button without selecting a row, you get the error you are seeing.
How to fix this is discussed at around 15:00m in the tutorial video.
You might also want to refer to the actual Web Tutorial document which has step-by-step instructions as well (Help->Tutorials->Web).