Angular CLI requires a current, active LTS, or maintenance LTS version of Node (https://angular.io/guide/setup-local#nodejs). which version 11 isn’t.
Node.JS 11 is no longer supported by the Node.Js team as after six months, odd-numbered releases (9, 11, etc.) become unsupported. More info: https://nodejs.org/en/about/releases
Solution : Upgrade Node version to 12+