You have to write those commands in the console:
composer require "components/font-awesome:5.4.*"
Reason for error
This is due to the fact that the flarum uses the Font Awesome icons, which are updated on a regular basis. At the last update, they renamed their icons from
Of course all their styles, that is:
The above commands turn this name back to the old one, because flarum/core is not yet adapted to the new file name. This will most likely change with the next update of the flarum.
@luceos or another administrator - I suggest you pin this post until you find another solution.