Example has minor incompatibility with Gulp 4.

[alex9000]

In your basic example you have:

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

In Gulp version 4 you have to pass gulp.series('sass') instead of ['sass'], the name of the task

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