Looks like the preprocessor for CSS/SCSS fails to generate a source map file (.map file) in addition to the compiled CSS for nucleus.
404 error media/gantry5/engines/nucleus/css-compiled/nucleus.css.map
media/gantry5/engines/nucleus/css-compiled folder shows 3 files but we should be expecting 4
Missing : nucleus.css.map
You can disable CSS source maps in chrome dev tools by: Going into your Chrome Dev Tools and in the upper RH corner click the little Gear icon. When the modal window opens make sure your on the General TAB -> Sources -> Enable CSS source maps and uncheck.
Disabling CSS source maps in chrome dev tools will "hide" the 404 notice, but is not a solution.
The source map file is a JSON file that defines a mapping between each generated CSS declaration and the corresponding line of the source file. Each CSS file contains an annotation that specifies the URL of its source map file, embedded in a special comment on the last line of the file. For example nucleus.css specifies sourceMappingURL=nucleus.css.map which is an expected designation from the CSS preprocessor.