Hi Setu,
Could you give us your source code as it seems that you are settng license file of aspose.cells using aspose.slides apis and your code must be wrong. Kindly use Aspose.Cells API to set Aspose.Cells license.
For reference, please check: http://www.aspose.com/documentation/file-format-components/aspose.cells-for-.net-and-java/licensing.html if it helps you.
Thank you.