Click on Apply and exit Local Security Policy.On the right panel, double-click on the option “ User Account Control: Admin Approval mode for the Built-in Administrators Account” and enable it.On the Local Security Policy window, navigate to Security Settings –> Local Policies –> Security Options.Press the Windows key + R to bring up the Run box, and then type secpol.msc.Or alternatively modify Windows security policy: You can either create a new user account in the Control Panel and then login as that user.
When you install a Windows 2012 R2 server, if you login as Administrator and try to run any Modern UI Apps (such as Internet Explorer) you’ll get this error message: “This app can’t be opened using the Built-in Administrator account please sign in with a different account and try again.” or “This app can’t be activated by the Built-in Administrator”.
If modern UI apps can't be opened using administrator account in windows 2012 R2 internet explorer, follow the article.