Cannot resolve directory 'bootstrap'

I've downloaded an angular starter and I'm trying to install Bootstrap on it. They explain how to do that in the External Stylesheet section.

I've installed bootstrap:

npm i bootstrap --save
npm i bootstrap-sass --save

and then added in the top of styles.scss:

@import 'bootstrap/scss/bootstrap.scss';

But it's marked with red mentions:

Cannot resolve directory 'bootstrap'

Any help will be profoundly appreciated!

via Alon