From the autocomplete guide:
Enable auto-complete-mode automatically for certain modes
auto-complete-mode will not automatically turn on for modes that are not in ac-modes . Therefore, you need to install, if necessary:
(add-to-list 'ac-modes 'brandnew-mode)
Or in your case
(add-to-list 'ac-modes 'lisp-mode)
For information only. The following modes are enabled by default.
(defcustom ac-modes '(emacs-lisp-mode lisp-interaction-mode c-mode cc-mode c++-mode java-mode clojure-mode scala-mode scheme-mode ocaml-mode tuareg-mode perl-mode cperl-mode python-mode ruby-mode ecmascript-mode javascript-mode js-mode js2-mode php-mode css-mode makefile-mode sh-mode fortran-mode f90-mode ada-mode xml-mode sgml-mode) "Major modes `auto-complete-mode' can run on." :type '(repeat symbol) :group 'auto-complete)
Oleg Pavliv
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