SET SERVEROUTPUT ON
DECLARE
v_student_id NUMBER := &sv_student_id;
v_section_id NUMBER := 89;
v_final_grade NUMBER;
v_letter_grade CHAR(1);
BEGIN
Your SELECT statement isn't finding the data you're looking for. That is, there is no record in the ENROLLMENT table with the given STUDENT_ID and SECTION_ID. You may want to try putting some DBMS_OUTPUT.PUT_LINE statements before you run the query, printing the values of v_student_id and v_section_id. They may not be containing what you expect them to contain.