Could not find module "@angular-devkit/build-angular" in angular?

When try to run ng serve --open command getting the above error.

If you face above issue while executing the ng serve  command, you can install the package as dev dependency. This is introduced in latest version of Angular 6.0.

 npm install --save-dev @angular-devkit/build-angular  

I faced the same issue and it resolved my problem and hope it will resolve your's too.