Oracle 18c

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CREATE TABLE mytable (Student_ID int, Year_of_paid_fee int);
Insert INTO mytable VALUES (101, 2018);
1 rows affected

Insert INTO mytable VALUES (101, 2019);
1 rows affected

Insert INTO mytable VALUES (101, 2021);
1 rows affected

Insert INTO mytable VALUES (102, 2018);
1 rows affected

Insert INTO mytable VALUES (102, 2021);

1 rows affected

with allyears as
(
    select 2018 yr from dual
    union all
    select 2019 from dual
    union all
    select 2020 from dual
    union all
    select 2021 from dual
)

select  mt.student_id , LISTAGG(distinct yr, ',') WITHIN GROUP (ORDER BY yr)  Year_of_Default
from mytable mt cross join allyears 
where not exists(select * from mytable m WHERE  mt.student_id=m.student_id and yr=m.YEAR_OF_PAID_FEE) 
group by mt.student_id

ORA-30482: DISTINCT option not allowed for this function

with allyears as
(
    select 2018 yr from dual
    union all
    select 2019 from dual
    union all
    select 2020 from dual
    union all
    select 2021 from dual
)
,cte as 
(
   select distinct  mt.student_id ,  yr
   from mytable mt cross join allyears 
   where not exists(select * from mytable m WHERE  mt.student_id=m.student_id and yr=m.YEAR_OF_PAID_FEE) 
)
select student_id, LISTAGG(yr,',') WITHIN GROUP (ORDER BY yr)  Year_of_Default
from cte 
group by student_id




STUDENT_ID
YEAR_OF_DEFAULT


101
2020

102
2019,2020