The following is a requirement when installing OpsMgr 2012, and here a quick guide on how to accomplish this.
Once you have a server prepared for a SCOM 2012 Web Console installation (can be the same server with all other features), open the IIS Manager and click on the server object.
On the left side, look for ‘ISAPI and CGI Restrictions’ and double click it.
You will then see a list of features – look for the ‘ASP.NET v4.x…’ you will need to set them to ‘Allowed’ by clicking the ‘Allow’ on the task windows (far left).
Although not a mandatory, I suggest restarting the IIS.