That's right - the net present value is a comparative number, so it requires that costs and benefits be expressed relative to another scenario.
You can use the Results view to show the NPV by showing Costs: Social Costs, and show Cumulative Values, then select Discounted Costs, and choose to show Differences vs. (your reference scenario of choice). Also be sure that you have selected the top branch in the tree with your mouse, to display all costs wherever they arise in your model. All of these options can be configured above the chart by selecting "More".
The (cumulative) result shown for the end year of your analysis will give you the NPV, or in other words, the cumulative discounted cost relative to another scenario.
Best,
Taylor