Hi lbck, thanks for posting on here too.
Unfortunately, according to the below article. the Drive Encryption component (which appears to be what you have installed) only supports up to MacOS 10.10.x
https://www.symantec.com/docs/TECH248333
This means you should either pursure the FileVault version, or go down the SEE route (which similarly uses FileVault).
The Quickstart guide for Symantec Encryption Desktop 10.4.2 for Mac FileVault, can be found below (it requires you enable FileVault2 first, and should be one of the options presented to you when installing SED on a Mac machine):
https://www.symantec.com/docs/DOC9328
While the install guide for the SEE route can be found below (NOTE: this includes first installing the SEE Management Server, in order to export the SEE for Mac Client package):
https://www.symantec.com/docs/DOC9134