Oracle docs (http://docs.oracle.com/cd/E25054_01/webcenter.1111/e10148/jpsdg_deploy.htm) says that:
Before deploying a WebCenter Portal application, you must create a WebLogic Managed Server based on the "Oracle WebCenter Custom Portal" template that contains all the required shared libraries and a MDS Repository.
Oracle does not recommend deploying WebCenter Portal applications to any of the three pre-configured Managed Servers created during the installation, or to a domain Administration Server.
Did you see "Oracle WebCenter Custom Portal" template while creating a domain? I don't see it.
Which managed server do you suggest to deploy webcenter portal application?
Just follow the advice in the following sentence: For instructions on how to create a new managed server, see "Extending an Existing Domain" in the Oracle Fusion Middleware Installation Guide for Oracle WebCenter.
It leads to: http://docs.oracle.com/cd/E21764_01/install.1111/e12001/config.htm#BABBHCGA which IMHO gives all the necessary details.