Org mode is still using the obsolete function `lisp-complete-symbol`,
but that function has been obsoleted in favor of `completion-at-point`
in version 24.4.  The attached diff is not meant to be installed, but to
show where the problem is.

The usual schedule for removing obsolete functions in Emacs would
suggest that `lisp-complete-symbol` will be removed in Emacs 31.1, but
we can't do that if Org mode is still using it.

Could you please look into this?