Assumption: Database 12c is installed successfully on a local VM that runs Oracle Enterprise Linux (OEL).
Issue: After a successful install of Oracle Database 12c, starting with the listener and the database, you might notice that the Enterprise Manager (EM) is not accessible in FireFox that comes with the standard Linux install on a VM by typing https://localhost:5500/em/login
Usually on Linux based VMs, FireFox does not come with the Flash plugin.
Resolution: Install the Flash plugin for FireFox or access the EM from the host machine.