Invalid number on this code in below query

You are using MAX(T1."BPLId") inside a scalar subquery within a CASE statement where T1."BPLId" is not aggregated correctly or is treated as non-numeric in an improper context.

You need to avoid aggregation in the WHERE clause. Since you're already grouping by T1."BPLId", you can just use T1."BPLId" directly like this:

SELECT 
    T0."TransId",
    T1."TransType",
    CASE
        WHEN T1."TransType" = '13' THEN (
            SELECT MAX(A0."LocCode") 
            FROM INV1 A0 
            WHERE A0."DocEntry" = T1."CreatedBy"
        )
        WHEN T1."TransType" = '1470000049' THEN (
            SELECT STRING_AGG(CAST(A11."ItemCode" AS NVARCHAR), ', ')
            FROM OITM A11
            WHERE A11."ItemCode" IN (
                SELECT DISTINCT A0."ItemCode" 
                FROM ACQ1 A0 
                WHERE A0."DocEntry" = T1."CreatedBy" 
                  AND A0."ObjType" = '1470000049'
            )
        )
        ELSE (
            SELECT MAX("Location") 
            FROM OWHS 
            WHERE "BPLid" = T1."BPLId"
        )
    END AS "Location"
FROM 
    OJDT T0  
    INNER JOIN JDT1 T1 ON T0."TransId" = T1."TransId"
    LEFT JOIN OACT T2 ON T2."AcctCode" = T1."Account"
WHERE 
    T1."TransType" = '1470000049'
GROUP BY 
    T0."TransId", T1."TransType", T1."Account", T1."BPLId", T1."CreatedBy"
ORDER BY 
    T1."TransType"