Oracle Group by-Case

You have to include the CASE logic in GROUP BY as well.

with shippedqty as (
  select sum(shipped_qty) as shpqty, source_salesitem_id
  from FACT_SHIPMENT_LINE
  where dim_org_id in (select dim_org_id from dim_org WHERE org_name like 'TSI%' AND org_name <> 'TSI FMS')
  group by source_salesitem_id
)
  select product.nsn_code,
  (CASE 
WHEN WHSE_CODE='EXE795' THEN 'TX'
WHEN WHSE_CODE='EXE796' THEN 'KUW'
WHEN WHSE_CODE<>'EXE795' AND WHSE_CODE<>'EXE796' THEN DLSF.whse_name

END) AS GAP,
        ven.vendor_name as Source,
        count(fsi.source_soline_id) as BO_num,
        sum(fsi.quantity - coalesce(sq.shpqty, 0)) as tirews_onBO, --qtyordered - qtydelivered
        min(od.THISDATE) as oldest_backorder
  from fact_salesitem fsi
  left join shippedqty sq on fsi.source_soline_id = sq.source_salesitem_id
  inner join dim_product product on fsi.dim_product_id  = product.dim_product_id
  inner join fact_pa_item pai on fsi.dim_product_id = pai.dim_product_id and pai.pa_isactive_ind = 'Y'
  inner join DIM_VENDOR ven on pai.DIM_VENDOR_ID = ven.DIM_VENDOR_ID
  left JOIN DIM_WHSE_BIN DLSF on FSI.DIM_WHSE_BIN_ID = DLSF.DIM_WHSE_BIN_ID
  INNER JOIN DIM_DATE od on FSI.CREATE_DATE_ID = od.DIM_DATE_ID
  INNER JOIN DIM_NAME_VALUES SOHS on FSI.DIM_NV_HDR_STATUS_ID = SOHS.DIM_NAME_VALUES_ID
  INNER JOIN DIM_NAME_VALUES SOLS on FSI.DIM_NV_LINE_STATUS_ID = SOLS.DIM_NAME_VALUES_ID
  WHERE FSI.DIM_ORG_ID IN (SELECT DIM_ORG_ID FROM DIM_ORG WHERE org_name like 'TSI%' AND org_name <> 'TSI FMS')
  and  SOLS.value_code in ('OH', 'BO') and  SOHS.value_code not in ('SC_AF')
  and product.nsn_code='2610-00-051-1270'
  group by product.nsn_code, (CASE 
WHEN WHSE_CODE='EXE795' THEN 'TX'
WHEN WHSE_CODE='EXE796' THEN 'KUW'
WHEN WHSE_CODE<>'EXE795' AND WHSE_CODE<>'EXE796' THEN DLSF.whse_name

END),ven.vendor_name;

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Definitely, there is data type mismatch in your case. Do you know which column is leading to this problem? If not, try to figure it out.

check the view dba_tab_columns and look for VARCHAR2 and NVARCHAR2 columns which you use in the where
user_tab_columns also works if it's all in the same schema

you'll have to case the varchar2 column to an nvarchar2

maybe it's column DLSF.whse_name

Tried Sharath code gave me the same error:
haracter set mismatch
12704. 00000 -  "character set mismatch"
*Cause:    One of the following
           - The string operands(other than an nlsparams argument) to an
           operator or built-in function do not have the same character
           set.
           - An nlsparams operand is not in the database character set.
           - String data with character set other than the database character
           set is passed to a built-in function not expecting it.
           - The second argument to CHR() or CSCONVERT() is not CHAR_CS or
           NCHAR_CS.
           - A string expression in the VALUES clause of an INSERT statement,
           or the SET clause of an UPDATE statement, does not have the
           same character set as the column into which the value would
           be inserted.
           - A value provided in a DEFAULT clause when creating a table does
           not have the same character set as declared for the column.
           - An argument to a PL/SQL function does not conform to the
           character set requirements of the corresponding parameter.
*Action:
Error at Line: 34 Column: 55

try this in the case:
cast DLSF.whse_name as varchar2(*length* ?)

Instead of running the whole query, run the CTE first. If no issue with CTE, JOIN one table at a time and try to find out where exactly you are getting the error.

The solution worked for me.