SELECT SUM(grades)
FROM
(
SELECT grade as grades
FROM "GradesTable"
 WHERE status='success' AND student_ID=1
 ORDER BY grade DESC
 LIMIT 50
) z

The problem here is the ORDER BY clause which you would know if you pasted the full error, instead of forcing us to recreate your problem from scratch because you didn't provide the DDL and data.

CREATE TABLE "GradesTable" ( status text, student_id int, grade int );
INSERT INTO "GradesTable" (status, student_id, grade) VALUES
  ('success', 1, 80),
  ('success', 1, 100);

Query

SELECT sum(grade)                                                    FROM "GradesTable"
 WHERE status='success' AND student_ID=1
 ORDER BY grade DESC
 LIMIT 50
;
ERROR:  column "GradesTable.grade" must appear in the GROUP BY clause or be used in an aggregate function
LINE 4:  ORDER BY grade DESC

However this works,

SELECT sum(grade)
 FROM "GradesTable"
 WHERE status='success' AND student_ID=1
 LIMIT 50
;
 sum 
-----
 180
(1 row)

But you shouldn't have a LIMIT if you have only one agg that can only ever return one row.

SELECT sum(grade)
 FROM "GradesTable"
 WHERE status='success' AND student_ID=1
;

If you meant to order by sum(grade) you can do that, but you lose the ability to order by grade the second you aggregate it together. However, in this example it doesn't matter because you're only returning one row.

And as a separate note you should never quote identifiers in PostgreSQL. Make everything lowercase, never quote table names or column names.