The installation of Symantec Endpoint Protection 12.1 RU5 (SEP) fails with the error "3-PAR signature failure, not running 3-PAR" when the installation is in interactive mode. When the installation is in silent mode, this error message will be logged in the following log file: C:\ProgramData\Symantec\Setup\Setup_s.log
This issue only occurs when the setting "Automatically uninstall existing security software" is enabled in the SEP client install settings.
This issue occurs when the root certificate "VeriSign Class 3 Public Primary Certification Authority - G5" is not present in the local computers Trusted Root Certificate Store. This certificate is used to verify the signatures of files used in the SEP installation.
There are two possible solutions to this issue.
Update Windows Root Certificates (Recommended)
The Microsoft KB Document
931125 explains how to update Window's list of root certificates. The update will contain the needed certificate.
Manually Download and Install the Missing Root Certificate