How to use co related subquery in oracle?

ORA-00979: not a GROUP BY expression
00979. 00000 -  "not a GROUP BY expression"
*Cause:    
*Action:
Error at Line: 13 Column: 29

ecoll_lodg_prod_code

select 
1 user_id,
t1.ecoll_cust_cr_acc_no  "ACCOUNT_NUMBER",
(case when t1.ecoll_lodg_prod_code='ENEFT' then 'NEFT'
when t1.ecoll_lodg_prod_code='RTGS' then 'RTGS'
when t1.ecoll_lodg_prod_code='IMPS' then 'IMPS'
when t1.ecoll_lodg_prod_code='ERTGS' then 'RTGS'
when t1.ecoll_lodg_prod_code='EFT' then 'FT'
when t1.ecoll_lodg_prod_code='NEFT' then 'NEFT' end) "COLLECTION_MODE",

(select sum(t2.ecoll_lodg_tran_amt) from ECOLL_LODG_VOL t2 where t2.ecoll_cust_cr_acc_no=t1.ecoll_cust_cr_acc_no 
and (trunc(t2.ecoll_lodg_date_time) between Last_Day(ADD_MONTHS(sysdate,-2))+1 and Last_Day(ADD_MONTHS(sysdate,-1)))
and t2.ecoll_lodg_prod_code=t1.ecoll_lodg_prod_code
) "PREVIOUSVALUE",

(select sum(ecoll_lodg_tran_amt) from ECOLL_LODG_VOL t where t.ecoll_cust_cr_acc_no=t1.ecoll_cust_cr_acc_no 
and  trunc(t.ecoll_lodg_date_time) BETWEEN TRUNC (LAST_DAY (ADD_MONTHS (SYSDATE,-1))+ 1)AND TRUNC(SYSDATE)
--and t.ecoll_lodg_prod_code=t1.ecoll_lodg_prod_code
)
"CURRENTVALUE",

(select count(t3.ecoll_lodg_tran_amt) from ECOLL_LODG_VOL t3 where t3.ecoll_cust_cr_acc_no=t1.ecoll_cust_cr_acc_no 
and (trunc(t3.ecoll_lodg_date_time) between Last_Day(ADD_MONTHS(sysdate,-2))+1 and Last_Day(ADD_MONTHS(sysdate,-1)))
-- and t3.ecoll_lodg_prod_code=t1.ecoll_lodg_prod_code
) "PREVIOUSVOLUME",

(select count(t4.ecoll_lodg_tran_amt) from ECOLL_LODG_VOL t4 where t4.ecoll_cust_cr_acc_no=t1.ecoll_cust_cr_acc_no 
and  trunc(t4.ecoll_lodg_date_time) BETWEEN TRUNC (LAST_DAY (ADD_MONTHS (SYSDATE,-1))+ 1)AND TRUNC(SYSDATE)
--and t4.ecoll_lodg_prod_code=t1.ecoll_lodg_prod_code
)"CURRENTVOLUME"

from 
ECOLL_LODG_VOL t1 where t1.ecoll_cust_cr_acc_no='111111111111111'
group by
1,
t1.ecoll_cust_cr_acc_no  ,
(case when t1.ecoll_lodg_prod_code='ENEFT' then 'NEFT'
when t1.ecoll_lodg_prod_code='RTGS' then 'RTGS'
when t1.ecoll_lodg_prod_code='IMPS' then 'IMPS'
when t1.ecoll_lodg_prod_code='ERTGS' then 'RTGS'
when t1.ecoll_lodg_prod_code='EFT' then 'FT'
when t1.ecoll_lodg_prod_code='NEFT' then 'NEFT' end);