If you are using older versions of Avast or AVG Antivirus, 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a compatibility hold that will prevent you from upgrading to Windows 10 1903 or Windows 10 1909 until you upgrade to a newer version of the antivirus software.
In a support notice to the Windows 10 Health Dashboard for Windows 10 1903 and 1909, Microsoft has stated that 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.