Gulp --production error - file cached?

Question

gulp.js

Please help me solve the problem. It's the first time I've come across this.
Installed a new Laravel project. Installed gulp and added gulpfile.js

var elixir = require('laravel-elixir');

elixir(function (mix) {
    mix.sass([
        "*.scss",
    ], 'public/css/app.css')
        .version('public/css/app.css');
});

I run gulp--production , an error occurs

File to import not found or unreadable: ~bootstrap/scss/bootstrap\n       Parent style sheet: C:/wamp64/www/simplysend/resources/assets/sass/app.scss\n        on line 4 of resources/assets/sass/app.scss\n>> @import \'~bootstrap/scss/bootstrap\';\n   ^\n\n

But there is no import on line 4. It was in the old version of the file. It's like the file is cached. How to fix?