When a report is first loaded up, there are several pre-selected columns that are listed there by defaults. Those default columns do not necessarily contain all the necessary requirements for each of the possible choices from the Add a chart section of the report. You will find that if you add a qualifying column (just as a test I added the Account ID) the Value field will then allow you to choose that column to populate the chart. So it is going to be a matter of finding the criteria you need to be in each "slice" of the chart, and ensuring the Value field will allow that column to be chosen.
I hope this fully explains this report and why the pie chart needs certain criteria. If you have any further questions on this, or if we can help in any other way, please let us know.