how to An unhandled exception occurred: Could not find module "@angular-devkit/build-angular" code example
Example: Could not find module "@angular-devkit/build-angular
ng update @angular/cli @angular/core
If you are updating from angular 7 to angular 8 then do this
ng update @angular/cli @angular/core
for more information read here https://github.com/just-jeb/angular-builders/blob/master/MIGRATION.MD