Could not load content for http://localhost:4200/assets/css/bootstrap.min.css.map: code example
Example: adminlte.min.css.map error
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 in bootstrap.min.css, bootstrap.min.js, popper.min.js or any other ccs file in which you are getting error.