Why did my network administrator turn on restricted mode?

For example, the DNS settings on your routers might be causing this, your network administrator might have this enabled at their end or if you installed a new add-on in your browser then it might be causing to force these settings and thus preventing the user to change this option.

How do I delete my administrator account on Windows 10?

Click on Manage other account. Enter the password for the administrator account if prompted. Click on the account which you want to delete (Microsoft admin account). Click on Delete the account.