Try like this:
Watch:
watch: {
livereload: {
files: [
'<%= yeoman.app %>/*.html',
'{.tmp,<%= yeoman.app %>}/styles/{,*/}*.css',
'{.tmp,<%= yeoman.app %>}/scripts/{,*/}*.js',
'<%= yeoman.app %>/img/{,*/}*.{png,jpg,jpeg,gif,webp,svg}'
],
tasks: ['livereload']
},
sass: {
files: "<%= yeoman.app %>/sass/{,**/}*.sass",
tasks: ["sass:tmp"]
}
}
Sass:
sass: {
tmp: {
files: {
"<%= yeoman.tmp %>/styles/index.css": "app/sass/index.sass",
},
options: {
paths: ["app/sass"]
}
},
}