From: http://windowsitpro.com/windows-server-2016/q-edge-can-t-be-opened-using-built-administrator-account

Q: I recently installed the Windows Server 2016 Tech Preview and when I went to use the new Edge browser to go the Internet I got the following message. “Microsoft Edge can’t be opened using the Built-in Administrator account. Sign in with a different account and try again.” Does Edge now have something like Internet Explorer’s Enhanced Security? This is a test system and I don’t want to bother making multiple accounts. How can I get my Windows Server 2016 test system connected to the Internet when I’m running as Administrator?

A: Fortunately Edge doesn’t have anything like the annoying Internet Explorer Enhanced Security. However, Microsoft has made Edge more like a true app and they have enhanced its security to restrict browsing for highly privileged accounts like the local administrator. There are a couple of workarounds. The simplest is just to use Internet Explorer (IE). IE is still present in Windows Server 2016 and you can start it by opening the Start menu and clicking on the Internet Explorer tile. IE security is still controlled by Internet Explorer Enhanced Security and it won’t display that error message.

Alternatively, if you want to use Edge you can change to the local security policies to allow Edge to run under the Administrator’s account. Use the search option in the taskbar and enter secpol.msc then click on it in the search results. Under Local Policies, Security Options navigate to User Account Control Admin Approval Mode for the Built-in Administrator account click on it to open the properties. Set the policy to Enabled and click Apply. After you reboot the system you will be able to use Edge under the local Administrator account.