There are two possible reasons:
  1. Check if your .map file is accessible publicly. For example, say your https://www.example.com/js/jquery.min.js file has the footer //# sourceMappingURL=../map/jquery.min.map Then check from your browser if https://www.example.com/js/../map/jquery.min.map is accessible.
  2. Make sure that you have a valid source map file. You can validate your map file in https://sourcemaps.io/