VARCHAR2和THE（4）之间存在错误，似乎是语法错误。

创建或替换

    FUNCTION Employee_exists
        (p_employee_id IN NUMBER)
    RETURN VARCHAR2(4);

    AS
    BEGIN
        SELECT employee_id
        FROM employees
        WHERE employee_id = p_employee_id;
        RETURN 'true';
    END Employee_exists;