Просто оберните ваш второй select
оператор подзапросом:
select avg("JUMLAH")
from(select d.nama,count(th.fk_distributor) as "JUMLAH"
from mh_distributor d join
th_beli th on th.fk_distributor = d.kode
group by d.nama
) t;
Итак, ваш полный оператор будет:
select d.kode, rpad(d.nama,75,' ') as "NAMA",
lpad(count(th.fk_distributor),10,' ') as "JUMLAH"
from mh_distributor d join
th_beli th
on th.fk_distributor = d.kode
having count(td.fk_produk) > (select avg("JUMLAH")
from(select d.nama,count(th.fk_distributor) as "JUMLAH"
from mh_distributor d join
th_beli th on th.fk_distributor = d.kode
group by d.nama
) t
)
group by d.nama, td.fk_produk, d.kode
order by d.kode asc;