FileMaker Server 14 is not yet expected to run on OS X 10.11 El Capitan. There is an updated expected to release this month (November) for compatibility. For more information see Knowledge Base Article #15187.
A couple things you might also verify:
-Library > FileMaker Server should not exist before starting the install (remove or rename an existing folder)
-Run the installer while logged in with a Local Admin User
-Make sure you are installing on a dedicated machine (no OS X Server, anti-virus, hosting sites, etc.)
I hope this information is of use.
Thank you for your reply. Bummer...