PostgreSQL Error: there is not unique constraint matching given keys for referenced table

I am making a database for registration of classes and I am having trouble with the "Enrollment" table. I need to pass all the primary keys from sections into enrollment, but I also want to pass an additional key ("sectionNumber"). However, I do not want to make "sectionNumber" part of the primary key for "Sections", because I don't want to be passing sectionNumber into every table that I have a foreign key in that is from "Sections." Anyone got any suggestions on how to fix this? I keep getting the error below.

-- HOLDS A SPECIFIC COURSE WITHOUT THE INSTANCES OF THE CLASS --
CREATE TABLE Courses (
    courseID      SERIAL      UNIQUE NOT NULL,
    department    TEXT               NOT NULL,
    courseNumber  VARCHAR(10)        NOT NULL,
    courseName    TEXT        UNIQUE NOT NULL,
    credits       INT                NOT NULL,
    PRIMARY KEY(courseID)
);

-- PEOPLE SUPERTYPE --
CREATE TABLE People (
    pid   SERIAL            UNIQUE NOT NULL,
    fname TEXT                     NOT NULL,
    lname TEXT                     NOT NULL,
    PRIMARY KEY(pid)
);

-- HOLDS THE DIFFERENT PROFESSORS TEACHING AT THE SCHOOL --
-- SUBTYPE OF PEOPLE --
CREATE TABLE Professors (
    professorID  INT  UNIQUE NOT NULL,
    status       TEXT        NOT NULL,
    CHECK(status = 'Full-Time' OR status = 'Part-time'),
    PRIMARY KEY(professorID),
    FOREIGN KEY(professorID) REFERENCES People(pid)
);

-- HOLDS THE SPECIFIC INSTANCES OF THE CLASS DEPENDING ON THE YEAR AND TERM --
CREATE TABLE Sections (
    courseID      INT          NOT NULL,
    year          INT          NOT NULL,
    term          TEXT         NOT NULL, 
    sectionNumber INT          NOT NULL,
    startDate     DATE         NOT NULL,
    endDate       DATE         NOT NULL,
    crn           INT          NOT NULL,
    CHECK(term = 'Fall' OR term = 'Winter' OR term = 'Spring' OR term = 'Summer'),
    PRIMARY KEY(courseID, year, term),
    FOREIGN KEY(courseID) REFERENCES Courses(courseID)
);

-- HOLDS THE EVENT OF THE CLASS --
-- A CLASS MAY HAVE DIFFERENT DAYS ON WHICH --
-- THEY MEET ON, SO THIS ALLOWS A CERTAIN --
-- SECTION TO HAVE SEVERAL DAYS WITHOUT CONFLICT --
CREATE TABLE ClassEvent (
    professorID   INT   UNIQUE NOT NULL,
    courseID      INT          NOT NULL,
    year          INT          NOT NULL,
    term          TEXT         NOT NULL,
    day           TEXT, 
    startTime     TIME,
    endTime       TIME,
    location      TEXT,
    campus        TEXT,
    CHECK(day = 'Monday' OR day = 'Tuesday' OR day = 'Wednesday' OR day = 'Thursday' OR day = 'Friday' OR day = 'Saturday' OR day = 'Sunday' OR day IS NULL),
    CHECK(term = 'Fall' OR term = 'Winter' OR term = 'Spring' OR term = 'Summer'),
    PRIMARY KEY(professorID, courseID, year, term, day, startTime, endTime),
    FOREIGN KEY(professorID) REFERENCES Professors(professorID),
    FOREIGN KEY(courseID, year, term) REFERENCES Sections(courseID, year, term)
);

-- GENERATES THE PREREQUESITES --
CREATE TABLE Prerequisites (
    courseID      INT        NOT NULL,
    year          INT        NOT NULL,
    term          TEXT       NOT NULL,
    prereqID      INT        NOT NULL,
    CHECK(term = 'Fall' OR term = 'Winter' OR term = 'Spring' OR term = 'Summer'),
    PRIMARY KEY(courseID, year, term, prereqID),
    FOREIGN KEY(courseID, year, term) REFERENCES Sections(courseID, year, term),
    FOREIGN KEY(prereqID) REFERENCES Courses(courseID)
);


-- HOLDS THE STUDENTS THAT WILL BE TAKING THE CLASSES --
-- SUBTYPE OF PEOPLE --
CREATE TABLE Students (
    studentID   INT  REFERENCES People(pid) UNIQUE NOT NULL,
    studentName TEXT                               NOT NULL,
    gradYear    DATE                        UNIQUE NOT NULL,
    PRIMARY KEY(studentID)
);

-- HOLDS A CLASS RECORD FOR STUDENTS (AND POSSIBLY PROFESSORS) --
CREATE TABLE Enrollment (
    studentID     INT  UNIQUE NOT NULL,
    courseID      INT         NOT NULL,
    year          INT         NOT NULL,
    term          TEXT        NOT NULL,
    sectionNumber INT         NOT NULL,
    grade         TEXT,
    CHECK(grade = 'A' OR grade = 'A-' OR grade = 'B+' OR grade = 'B' OR grade = 'B-' OR grade = 'C+' OR grade = 'C' OR grade = 'C-' OR grade = 'D+' OR grade = 'D' OR grade = 'D-' OR grade = 'F' OR grade = 'P/F'),
    CHECK(term = 'Fall' OR term = 'Winter' OR term = 'Spring' OR term = 'Summer'),
    PRIMARY KEY(studentID, courseID, year, term, sectionNumber),
    FOREIGN KEY(studentID) REFERENCES Students(studentID),
    FOREIGN KEY(courseID, year, term) REFERENCES Sections(courseID, year, term),
    FOREIGN KEY(sectionNumber) REFERENCES Sections(sectionNumber)
);

-- HOLDS THE DIFFERENT DEGREES THAT CAN BE ATTAINED AT THE COLLEGE/UNIVERSITY --
CREATE TABLE Degrees (
    degreeID     SERIAL       UNIQUE NOT NULL,
    degreeName   TEXT                NOT NULL,
    degreeType   TEXT                NOT NULL,
    degDepartment VARCHAR(4)          NOT NULL,
    CHECK(degreeType = 'Major' OR degreeType = 'Minor' OR degreeType = 'Masters'),
    PRIMARY KEY(degreeID)
);

-- HOLDS THE CLASSES THAT WILL MAKE UP A DEGREE --
CREATE TABLE DegreeReq (
    degreeID INT REFERENCES Degrees(degreeID) UNIQUE NOT NULL,
    courseID INT REFERENCES Courses(courseID)        NOT NULL,
    PRIMARY KEY(degreeID, courseID)
);

-- HOLDS THE INSTANCE OF A DEGREE FOR A CERTAIN STUDENT --
-- FOR EXAMPLE: A STUDENT CAN HAVE A MAJOR AND A MINOR --
-- SO HE/SHE CAN STORE THEM SEPARATELY --
CREATE TABLE DegreeInstance (
    degreeID        INT  REFERENCES Degrees(degreeID)   UNIQUE NOT NULL,
    studentID       INT  REFERENCES Students(studentID) UNIQUE NOT NULL,
    startDate       DATE                                       NOT NULL,
    endDate         DATE                                       NOT NULL,
    creditsRequired INT                                        NOT NULL, 
    PRIMARY KEY(degreeID, studentID)
);

-- HOLDS ALL THE RATE MY PROFESSOR STATS --
CREATE TABLE Rating (
    professorID      INT        UNIQUE NOT NULL,
    rmpID            BIGINT     UNIQUE NOT NULL,
    avgRating        FLOAT             NOT NULL,
    avgHelpfulness   FLOAT             NOT NULL,
    avgClarity       FLOAT             NOT NULL,
    avgEasiness      FLOAT             NOT NULL,
    PRIMARY KEY(professorID, rmpID),
    FOREIGN KEY(professorID) REFERENCES Professors(professorID)
);

ERROR:  there is no unique constraint matching given keys for referenced table "sections"
********** Error **********

ERROR: there is no unique constraint matching given keys for referenced table "sections"
SQL state: 42830