Because creating the Output Report can take quite a bit of time, and the amount of time increases as the number of Analysis Fields and Number of Groups increases, no report is created when you specify more than 15 groups or more than 15 fields. This upper limit can be altered, however.
If you want to generate a report in cases where you have more than 15 Analysis Fields or when you specify more than 15 groups, follow the instructions outlined in the tool documentation. Because creating the report does take quite a bit of time, you might want to run this tool without creating a report until you settle on a final list of Analysis Fields. Keep in mind that the graphics in the report file are more difficult to interpret when there are lots of groups and/or many Analysis Fields.