I want to customize
org-agenda-prefix-format before org agenda starts ：
(add-to-list 'org-agenda-prefix-format '(todo . " %b"))
But if I put the previous line in my .emacs.d, Emacs will show me this error:
Error (use-package): org :config: Symbol's value as variable is void: org-agenda-prefix-format
It seems that the variable is not defined before org agenda has been started once. How to solve this problem?