In the challenge Bootstrap: Add Font Awesome Icons to our Buttons the instructions appear to have a typo.
The instructions state:
Use Font Awesome to add a
thumbs-upicon to your like button by giving it an
ielement with the classes
fa-thumbs-up. Make sure to keep the text “Like” next to the icon.
The instructions should indicate that you change the class to fas, not fa. On second look the test and the introductory example include fas, but the typo may be initially confusing to others as it was to me.