Update to Gulp 4.0, task build error

Can someone help? I am trying to update to Gulp 4.0  but I get an error when running “gulp watch”. What am I doing wrong?

AssertionError [ERR_ASSERTION]: Task never defined: build

The relevant code in my Gulp file is :

// Reload all Browsers  
gulp.task('bs-reload', gulp.series(function () {
browserSync.reload();  
}));  
  
// Browser-Sync task  
gulp.task('server', gulp.series('build', function () {
browserSync.init(["css/\*.css", "js/\*.js"], {
server: {baseDir:"./" , port:80}
// If you use vhosts use the line below and comment out the line above.  
 //proxy: "demo.local"  
});  
}));  

where is  you build task https://www.joezimjs.com/javascript/complete-guide-upgrading-gulp-4/