Hi,
Thank you for posting in Symantec community.
Could you please attach the screenshot?
Does it ask for user-name and password when you tries to add computer? If yes make sure to provide admin credentials.
If able to add computer successfully but package is not getting deployed then need to refer the following articles.
Steps to prepare computers to install Symantec Endpoint Protection 12.1 client
http://www.symantec.com/docs/TECH163112
Preparing Windows operating systems for remote deployment
http://www.symantec.com/docs/HOWTO80805