我创建了两个表：

create table
exams (
exams_id bigint primary key generated always as identity,
name text,
created_at timestamptz default now()
);

和/或

create table
tutors (
tutors_id bigint primary key generated always as identity,
name text,
email text,
created_at timestamptz default now()
);

然后我添加外键如下：

alter table
if exists public.exams 
add column tutor_id bigint;

和/或

alter table
if exists public.exams
add foreign key(tutor_id) references tutors(tutor_id);

最后一个命令抛出以下错误：

column "tutor_id" referenced in foreign key constraint does not exist

有人能告诉我，我做错了什么吗？