错误信息告诉你一切.你需要从触发器函数中做一个 RETURN :

The error message tells you all. You need to do a RETURN from the trigger function:

CREATE OR REPLACE FUNCTION forest_aud_func() returns trigger as $tree_stamp$ 
BEGIN
    insert into Audit values('k',124,'l');
    return new;
END;
$tree_stamp$
LANGUAGE plpgsql;

来自手册:

触发器函数必须返回 NULL 或记录/行值，该值与触发触发器的表的结构完全相同.