How do i...make a trigger that moves elements from a table to another one

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PostgreSQL

Hello I have a question: I am doing a trigger for some tables.

The trigger has to move the students with 3 grades(not F) to another table called diploma.

The thing is the trigger isn't working and I don't know how to fix this. please enlighten me.

What I have tried:

CREATE TABLE GRADE
(
   STUDENTNUMBER   INT NOT NULL,
   SECTIONID       INT NOT NULL,
   GRADE           CHAR (1),
   PRIMARY KEY (SECTIONID, STUDENTNUMBER),
   FOREIGN KEY (STUDENTNUMBER) REFERENCES STUDENT
);

CREATE TABLE STUDENT
(
   STUDENTNAME     VARCHAR (30) NOT NULL,
   STUDENTNUMBER   INT NOT NULL,
   CLASS           VARCHAR (30),
   SECTIONID       INT[] NOT NULL,
   MAJOR           VARCHAR (20),
   PRIMARY KEY (STUDENTNUMBER)
);


CREATE FUNCTION test() RETURNS TRIGGER AS $$
DECLARE
  x INTEGER;
BEGIN
	SELECT COUNT(GRADE) INTO x FROM GRADE, student WHERE GRADE.STUDENTNUMBER = student.STUDENTNUMBER;
	IF (X >= 3) THEN
		INSERT INTO diploma (STUDENTNAME,STUDENTNUMBER , CLASS,SECTIONID,MAJOR) values
			(SELECT student.STUDENTNAME, student.STUDENTNUMBER, student.CLASS, student.SECTIONID, student.MAJOR);
  	END IF;
  RETURN x;
END;
$$ LANGUAGE plpgsql;


CREATE TRIGGER UPDATE_TEST
	AFTER INSERT OR UPDATE ON GRADE
	FOR EACH STATEMENT EXECUTE PROCEDURE
	test();