"sass" set up problem

I am trying to use vuetify so I did the instructions
Get started with Vuetify — Vuetify
Then I have the error Preprocessor dependency “sass” not found. Did you install it?
I tried to install it with npm install sass.
I saw the sass folder at node_modules and the “sass”: “^1.51.0” at dependencies (package.json) but I have again the error “Preprocessor dependency “sass” not found. Did you install it?”
What is wrong?

I restart the server and the problem solved.
Now I am trying to use this example without success at my App.vue , I use the at my template and I use " import vuetify from ‘@/plugins/vuetify’ " at the script section.
How can I set up vuetify at a .vue script?
This is the example I follow