Check that you have the latest gulp-sass version in package.json; it’s a dependency of node-sass + required by Ionic, e.g.
"dependencies": {
"gulp-sass": "^2.3.2"
},
Uninstall gulp-sass
$ npm uninstall --save-dev gulp-sass
Reinstall gulp-sass
$ npm install --save-dev gulp-sass@2
There are a large number of issues pertaining to node-sass installation.
You may have to rebuild node-sass: $ npm rebuild node-sass