Gulp has been updated to 4.0.0, and 3.9.1 contains the now deprecated dependency gulp-util.
Upgrading can be done with the following guide:
https://www.liquidlight.co.uk/blog/article/how-do-i-update-to-gulp-4/
'run-sequence' is also no longer needed because of gulp.series and gulp.parallel functions that have the same functionality.