There are two possible reasons:
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.Make sure that you have a valid source map file. You can validate your map file in
https://sourcemaps.io/