Microsoft has placed a compatibility hold that will prevent you from upgrading to Windows 10 1903 or Windows 10 1909 until you upgrade to a newer version of the antivirus software. Users who have AVG or Avast Antivirus versions 19.5.4444.567 or lower installed will be prevented form installing the May 2019 Update and the November 2019 Update. Microsoft and Avast have also released support bulletins [Avast, AVG] with similar information as the one published by Microsoft. After upgrading to the latest version of AVG, users can then upgrade to the new version of Windows 10, version 1909.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-upgrades-blocked-if-using-old-versions-of-avg-avast/