Why does Emacs say ‘Key sequence XXX uses invalid prefix characters’?

Usually, one of two things has happened. In one case, the control character in the key sequence has been misspecified (e.g., ‘C-f’ used instead of ‘\C-f’ within a Lisp expression). In the other case, a prefix key in the keystroke sequence you were trying to bind was already bound as a complete key. Historically, the ‘ESC [’ prefix was usually the problem, in which case you should evaluate this form before attempting to bind the key sequence:

(keymap-global-unset "M-[")