SEP SBE is end of support life. The latest (and last) version for it is 12.1.5.
Otherwise, you now need to move to the cloud version.
Full details at these links:
End of Life for Symantec Endpoint Protection Small Business Edition (SEP SBE) 12.1.X and Symantec Protection Suites Small Business Edition (SPS SBE) 4.0
http://www.symantec.com/docs/DOC8217
https://www.symantec.com/page.jsp?id=sbe-migration