Why is gulp watch not starting?

Question

gulp.js

var gulp = require('gulp'),
  sass = require('gulp-sass'),
    watch = require('gulp-watch');

gulp.task('sass', function() {
  return gulp.src('app/scss/**/*.scss')
  .pipe(sass())
  .pipe(gulp.dest('app/css'));
});  

gulp.task('watch', function() {
    gulp.watch('app/scss/**/*.scss', ['sass']);
});

Answer 1

It looks like you are using Gulp 4.
In that case, you misspelled the "watch" task.
Replace

gulp.task('watch', function() {
    gulp.watch('app/scss/**/*.scss', ['sass']);
});

On the

gulp.task('watch', function() {
    gulp.watch('app/scss/**/*.scss', gulp.parallel('sass'));
});

Or

gulp.task('watch', function() {
    gulp.watch('app/scss/**/*.scss', gulp.series('sass'));
});

Answer 2

Problem solved