Source map files, like bootstrap.min.css.map, are generated automatically during files minification/compiling process. They actually are not needed for the production, but at the same time occupy 3 times more disk space than ones which they are mapping.
If you want just to remove that message, you can either remove comments like /*# sourceMappingURL=bootstrap.min.css.map */ in bootstrap.min.css, bootstrap.min.js, popper.min.js or any other ccs file in which you are getting error.