Fix for disabled New Web Application button in SharePoint 2013
Sometimes after a new installation of SharePoint 2013 on a Windows 2008 R2 system you will see that the New Web Application button is in central administration is disabled even when you have logged in as system account.
The fix for enabling the new web application button is to turn off the notification settings for the system account in the windows server. Below are the steps for turning off notification
1.Open System and security in control panel
2.Under Action center click "Change user account control settings".
3.Move the scroll bar to never notify and click Ok.
4.Restart the server.
After the system has restarted and you navigate to the create web application screen you will now see that the new web application button will be enabled.