The <icon> element is built around the Font Awesome font collection. The fontawesome package contains all the necessary font files (for use with both pdflatex and xelatex), and a style file that interfaces, in a semi-elaborate way, with the font files. If you have installed this package, and the fonts still seem unavailable, figure out how to adjust your setup or your system configuration. Simply installing a font file, such as the more recent Font Awesome 5 is not going to work. See Chapter 37 for more about fonts in .