Error - Incorrect Syntax

SELECT  
    T0.DOCNUM,
    T0.DOCENTRY,
    T1.LineNum,
    T1.ITEMCODE,
    (
        SELECT R4.TaxSum 
        FROM INV4 R4 
        WHERE R4.staType IN ('-100') 
        AND R4.RelateType = 1 
        AND R4.DocEntry = T0.DocEntry 
        AND R4.LineNum = T1.LineNum
    ) AS CGSTAMNT
FROM     
    OINV T0 
    INNER JOIN INV1 T1 ON T0.DOCENTRY = T1.DOCENTRY;

Double single quotes are typically used to escape a single quote character within a SQL string, but it seems like you have extra single quotes in your query. You should adjust your query to properly use single quotes where needed.

SELECT  
    T0.DOCNUM,
    T0.DOCENTRY,
    T1.LineNum,
    T1.ITEMCODE,
    (
        SELECT R4.TaxSum 
        FROM INV4 R4 
        WHERE R4.staType IN ('-100') 
        AND R4.RelateType = 1 
        AND R4.DocEntry = T0.DocEntry 
        AND R4.LineNum = T1.LineNum
    ) AS CGSTAMNT
FROM     
    OINV T0 
INNER JOIN 
    INV1 T1 ON T0.DOCENTRY = T1.DOCENTRY