You try to use the install.sh -c option to verify the signature of the RPM package with which you were provided, on a Linux computer. 

You receive the following output: 

Checking signature on sep.rpm
sep.rpm signature verification FAILED
Packages signature serification failed.

This occurs when the Symantec Endpoint Protection for Linux GPG public key has not been installed on the computer, or if you are using a version that is not signed, in which the -c switch is a legacy feature. 

Use the instructions in the Verifying the signature of RPM packages included with SEP 14.2 MP1 and later knowledge base article (article 172965) to install the public key. 

For version 14.3, the RPMs are not signed, but the legacy -c switch is still in the install.sh script. For these systems, you can verify the RPMs by running the following command from the directory into which you unzipped the SEP install: 

rpm --checksig Repository/*.rpm