再帰で呼び出す関数名が間違ったままだった・・・
(defun two-times-every-node(tree)
(cond ((null tree) tree)
((atom tree) (* tree 2))
(t (cons (two-times-every-node (car tree)) (two-times-every-node (cdr tree))))))
(print (two-times-every-node '(1 (2 5 (10 30)) 3)))