You are probably running Mendix 8.
The location of the sass is changed.
In the Gulpfile.js you have to change the following line. The web part need to be added to the path.
var sourceStyleFolder = 'theme/styles/web';
“Since your task might contain asynchronous code you have to signal gulp when your task has finished executing (= "async completion").”
You could try to use Calypso if that fits your needs. Calypso will work with mendix 7 & 8.