Oracle 21c

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-- made-up data to replicate some intermediate output
create table GL_JOURNAL_LINES (customer_no, account_no, accounted_dr, accounted_cr, effective_date) as
select 5010011, 19, 140, 0, date '2021-01-01' from dual union all
select 5010011, 19, 1.23, 0, date '2021-01-02' from dual union all
select 5010011, 19, 8, 0, date '2021-02-01' from dual union all
select 5010011, 19, 8, 0, date '2021-02-02' from dual union all
select 5010011, 27, 130500, 0, date '2021-02-01' from dual
5 rows affected

-- original query from start of question
select customer_no, account_no, sum(accounted_dr), sum(accounted_cr), to_char(effective_date, 'YYYY-MM') as "Effective Months and Years"
from GL_JOURNAL_LINES
group by account_no, customer_no, to_char(effective_date, 'YYYY-MM')
order by to_char(effective_date, 'YYYY-MM') asc


CUSTOMER_NO
ACCOUNT_NO
SUM(ACCOUNTED_DR)
SUM(ACCOUNTED_CR)
Effective Months and Years


5010011
19
141.23
0
2021-01

5010011
19
16
0
2021-02

5010011
27
130500
0
2021-02


-- original pivot query
select * from(select customer_no, account_no, sum(accounted_dr), sum(accounted_cr), to_char(effective_date, 'YYYY/MM') as "Effective Months and Years" from GL_JOURNAL_LINES)
pivot(sum(accounted_dr), sum(accounted_cr) for to_char(effective_date, 'YYYY-MM') in ("2021/01", "2021/02", "2021/03", "2021/04", "2021/05", "2021/06"));
order by to_char(effective_date, 'YYYY-MM')
ORA-01738: missing IN keyword

select * from (
  select customer_no, account_no, accounted_dr, accounted_cr,
    trunc(effective_date, 'MM') as effective_month
  from GL_JOURNAL_LINES
)
pivot (
  sum(accounted_dr) as dr, sum(accounted_cr) as cr
  for effective_month
  in (
    date '2021-01-01' as "2021/01", date '2021-02-01' as "2021/02",
    date '2021-03-01' as "2021/03", date '2021-04-01' as "2021/04",
    date '2021-05-01' as "2021/05", date '2021-06-01' as "2021/06"
  )
)


CUSTOMER_NO
ACCOUNT_NO
2021/01_DR
2021/01_CR
2021/02_DR
2021/02_CR
2021/03_DR
2021/03_CR
2021/04_DR
2021/04_CR
2021/05_DR
2021/05_CR
2021/06_DR
2021/06_CR


5010011
19
141.23
0
16
0
null
null
null
null
null
null
null
null

5010011
27
null
null
130500
0
null
null
null
null
null
null
null
null


select * from (
  select customer_no, account_no, accounted_dr, accounted_cr, 
    to_char(effective_date, 'YYYY-MM') as effective_month
  from GL_JOURNAL_LINES
)
pivot (
  sum(accounted_dr) as dr, sum(accounted_cr) as cr
  for effective_month
  in (
    '2021-01' as "2021/01", '2021-02' as "2021/02", '2021-03' as "2021/03",
    '2021-04' as "2021/04", '2021-05' as "2021/05", '2021-06' as "2021/06"
  )
)


CUSTOMER_NO
ACCOUNT_NO
2021/01_DR
2021/01_CR
2021/02_DR
2021/02_CR
2021/03_DR
2021/03_CR
2021/04_DR
2021/04_CR
2021/05_DR
2021/05_CR
2021/06_DR
2021/06_CR


5010011
19
141.23
0
16
0
null
null
null
null
null
null
null
null

5010011
27
null
null
130500
0
null
null
null
null
null
null
null
null

CUSTOMER_NO	ACCOUNT_NO	SUM(ACCOUNTED_DR)	Effective Months and Years
5010011	19	141.23	2021-01
5010011	19	16	2021-02
5010011	27	130500	2021-02

CUSTOMER_NO	ACCOUNT_NO	2021/01_DR	2021/01_CR	2021/02_DR	2021/02_CR	2021/03_DR	2021/03_CR	2021/04_DR	2021/04_CR	2021/05_DR	2021/05_CR	2021/06_DR	2021/06_CR
5010011	19	141.23	0	16	0	null	null	null	null	null	null	null	null
5010011	27	null	null	130500	0	null	null	null	null	null	null	null	null