Solution
If you switch the line chart display to stacked lines and the above message is shown, it means that one or more list values of the objects in your chart has a negative value and/or the objects do not all have the same number of list values.
An example of a "problematic" line chart is shown below.

As can be seen, Object A has three values (along the x-axis) instead of four, like the other objects. Additionally, Object C has one value that is below zero. Therefore, displaying stacked lines is not possible.
In the following chart all list values are positive, and the number of list values is the same for each object. Stacked lines can be displayed:
