Make your Windows root certs are updated, see here:
Certificate error occurs when attempting to install or upgrade Symantec Endpoint Protection
http://www.symantec.com/docs/TECH218029
Symantec Endpoint Protection 12.1 RU4 installation or migration on Windows fails due to outdated root certificate
http://www.symantec.com/docs/TECH217284
Try the install after updating them