Please post the full SEP_INST.log and SIS_INST.log files. Have you run cleanwipe? Please try it.
What logs do I need to gather in order to troubleshoot a failed SEP 12.1 client installation?
http://www.symantec.com/docs/TECH164067
See if these apply:
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Symantec Endpoint Protection 12.1 RU4 installation or migration on Windows fails due to outdated root certificate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