错误代码：1824。无法打开引用表“级别”

Question

  

错误代码：1824。无法打开引用的表“级别”

create table users
(
user_id int  auto_increment  primary key  not null unique ,
username varchar(50)    unique null ,
email varchar(50)   unique  ,
passwords varchar(50) not null,
gender varchar(50) ,
login_status boolean not null  
);


create table category (
 category_id int  primary key not  null auto_increment unique ,
 category_name varchar(50) not null,
 category_authour varchar(50) not null,
 KEY category_name_index (category_name)
 );
 create table theme(
 id int primary key not null unique ,
 image_name  varchar(50) not null
-- level_number int,
-- foreign key (level_number) references levels (level_number)
 );
create table questions  ( 
question_id int primary key not null unique ,
 category_name varchar(50) not null,
 question_authour varchar(50) not null,
content varchar(50) not null ,
answer boolean  not null,
level_number int,

 KEY content_index (content),
 KEY question_id_index (question_id),
 foreign key (category_name) references category (category_name),
 foreign key (level_number) references levels  (level_number)
 );


 create table friends (
username int  not null,
score  int primary key not null,
category_name varchar(50) not null,    
 foreign key (question_id) references questions (question_id),
 foreign key (score) references users_scores  (score),
 foreign key (category_name) references category (category_name)
 );
 create table levels  (  
level_id int not null primary key ,
level_number int,
KEY level_number_index (level_number)
  );