I see. Maybe I should try this process myself so I can see each step in action. From your screenshot of the Append Points table (result of step 9?), it looks like all of the line_points were appended to the append_points with null values.
And I apologize as I just sort of talk this out for myself, but I guess when you are trying to run the points to line tool in step 10 the tool is failing because of those null values. So it tells you that there aren't enough points to create lines.
Does your Append_Points feature plot those null value features on the map? Or do you only see the original 121 point features when you have that layer showing on your map? Sorry I'm not more help here.
Casey Carpenter