select failed 3504 selected non-aggregate values must be part of the associated group - response (1) by dnoeth

It depends on the expected result set.
You might have to put all other columns in your select list in a GROUP BY 1,3,4,5,6,7,8,9 or you might need a MAX(b.Effective_Dt) OVER (PARTITION BY whatever).
 
When i look at your query i see several possible improvements:
- There's a lot of DISTINCTs, i doubt they're actually neccessary. When the optimizer is not able to eliminate them during optimization (e.g. GROUP BY already resturns unique rows) it's a lot of overhead. 
- The MAX(date) selects to find the latest row can be replace with a simple RANK/ROW_NUMBER:

    Select Distinct
      a.GROUP_CUSTOMER_ID
      ,Max(EXPIRATION_DT) ExpDat
    From
      PSOR_Temp.PRODUCER_X_GROUP a
    Group by
      1
  ) Grp
  Inner Join
  (
    Select Distinct
      a.GROUP_CUSTOMER_ID
      ,a.PRODUCER_ID
      ,a.EXPIRATION_DT ExpDat
    From
      PSOR_Temp.PRODUCER_X_GROUP a

with:

    Select Distinct -- probably not needed
      a.GROUP_CUSTOMER_ID
      ,a.PRODUCER_ID
      ,a.EXPIRATION_DT ExpDat
    From
      PSOR_Temp.PRODUCER_X_GROUP a
    QUALIFY RANK() 
            OVER (PARTITION BY GROUP_CUSTOMER_ID 
                  ORDER BY ExpDat DESC) = 1

 
 
Dieter