assignment_code needs to be unique but I dont want it to - sql

Question

I have the following table
CREATE TABLE assignment (
assignment_code VARCHAR(10),
course_code VARCHAR(10),
PRIMARY KEY(course_code, assignment_code),
FOREIGN KEY(course_code) REFERENCES course(course_code));
But when I try to create it PostgreSQL tells me assignment_code needs to be unique. However my assignment is to make assignment_code unique per course in stead of completely unique, how do I do that?
the code below creates the enitre database (no data)
CREATE TABLE students (
student_id BIGINT PRIMARY KEY,
first_name VARCHAR(15),
surname VARCHAR(35),
enrollment_year INT,
bsn BIGINT,
class VARCHAR(5),
Graduated BOOLEAN);
CREATE TABLE teachers (
bsn BIGINT PRIMARY KEY,
first_name VARCHAR(15),
surname VARCHAR(35),
salary REAL,
scale INT,
CONSTRAINT salary CHECK(salary < (25 * scale) AND salary > (20 * scale)));
CREATE TABLE course (
course_code VARCHAR(10) PRIMARY KEY,
course_name VARCHAR(20),
course_follower VARCHAR(5),
study_points INT,
Amount_of_assignments INT);
CREATE TABLE teaches (
bsn BIGINT,
class VARCHAR(5),
course_code VARCHAR(10),
PRIMARY KEY(bsn, class, course_code),
FOREIGN KEY(bsn) REFERENCES teachers(bsn),
FOREIGN KEY(course_code) REFERENCES course(course_code));
CREATE TABLE study_program (
course_name VARCHAR(20),
course_code VARCHAR(10) PRIMARY KEY,
level VARCHAR(15),
duration VARCHAR(10),
FOREIGN KEY(course_code) REFERENCES course(course_code));
CREATE TABLE assignment (
assignment_code VARCHAR(10),
course_code VARCHAR(10),
PRIMARY KEY(course_code, assignment_code),
FOREIGN KEY(course_code) REFERENCES course(course_code));
CREATE TABLE records (
student_id BIGINT,
course_code VARCHAR(20),
school VARCHAR(50),
PRIMARY KEY(student_id, course_code),
FOREIGN KEY(student_id) REFERENCES students(student_id),
FOREIGN KEY(course_code) REFERENCES study_program(course_code));
CREATE TABLE make (
student_id BIGINT,
assignment_code VARCHAR(20),
completed BOOLEAN,
PRIMARY KEY(student_id, assignment_code),
FOREIGN KEY(student_id) REFERENCES students(student_id),
FOREIGN KEY(assignment_code) REFERENCES assignment(assignment_code));
CREATE TABLE prerequisit (
assignment_code VARCHAR(20),
prerequisit_code VARCHAR(20),
course_code VARCHAR(20),
PRIMARY KEY(assignment_code, course_code),
FOREIGN KEY(assignment_code) REFERENCES assignment(assignment_code),
FOREIGN KEY(course_code) REFERENCES course(course_code),
FOREIGN KEY(prerequisit_code) REFERENCES assignment(assignment_code));
CREATE TABLE records_2 (
assignment_code VARCHAR(20),
course_code VARCHAR(20),
bsn BIGINT,
mandatory BOOLEAN,
year INT,
week INT,
PRIMARY KEY(assignment_code, course_code, bsn),
FOREIGN KEY(assignment_code) REFERENCES assignment(assignment_code),
FOREIGN KEY(course_code) REFERENCES course(course_code),
FOREIGN KEY(bsn) REFERENCES teachers(bsn));
CREATE TABLE designes (
course_code VARCHAR(15),
bsn BIGINT,
PRIMARY KEY(course_code),
FOREIGN KEY(course_code) REFERENCES course(course_code),
FOREIGN KEY(bsn) REFERENCES teachers(bsn));
CREATE TABLE reviews (
bsn BIGINT,
course_code VARCHAR(15),
PRIMARY KEY(bsn, course_code),
FOREIGN KEY(bsn) REFERENCES teachers(bsn),
FOREIGN KEY(course_code) REFERENCES course(course_code));

Answer 1

Lurking deep in your code is this:
CREATE TABLE records_2
. . .
FOREIGN KEY(assignment_code) REFERENCES assignment(assignment_code),
A foreign key reference needs to be to a unique or primary key. If you have a composite primary key, then you need to use all the keys. Hence, this reference is missing course_code.
I am a strong believer in synthetic primary keys -- that is, serial integer columns that provide this information. I would recommend that you use them in your table and for foreign key references.