Hi Arnab,
This is Jennifer from IObit Software. Sorry for any inconveniences it caused to you! The license is valid in IObit Malware Fighther v6, could you check the version you use?
You could download the v6 installer from http://update.iobit.com/dl/cus/imfv6-setup-gmbd.exe or from https://www.iobit.com
License: 9B565-32EED-F0E80-51764