I have uninstalled Avast Antivirus Free. How can I turn on Windows Defender? It is not running now. Windows 10 Pro (1809).
First have you rebooted after uninstalling Avast ?
Windows 10 should have restarted it automatically, in the same way that it disabled it when you install a third party antivirus (in this case Avast).
Have you gone to the Windows 10, 'Windows Defender Security Center' that should surely indicate what antivirus you have installed or not ?