Hi @AnoopRawat 

Please try to install it from developer portal (https://developer.servicenow.com/dev.do#!/manage-instance )

If  from there only you are getting activation failure message , 

Sometimes, the background process times out or the status tracker gets stuck, meaning the plugin is actually active:
• Log into your PDI and type sys_plugin.list in the Application Navigator.
• Search for Discovery (com.snc.discovery) to check its state.
• If it is listed, open the record and click  Repair if available.

• Search for Service Mapping (com.snc.service-mapping) to check its state.
• If it is listed, open the record and click  Repair if available.