I modified what you wrote and it works
INSERT INTO WTr1 (LineNum, DocEntry, ItemCode, Dscription, WhsCode, Quantity, U_ClientDesc)
select MAX(T1.LineNum)+1, T1.DocEntry, '624262301875', 'Console 5.99', 'CONS', '30', 'Console 5.99'
FROM OWTR T0 INNER JOIN WTR1 T1 ON T0.DocEntry = T1.DocEntry
WHERE T0.[DocNum] >= '914141' AND T0.[DocNum] <= '914201'
Group By T1.DocEntry
SELECT T0.[DocNum], T0.[DocDate], T0.[CardCode], T0.[CardName], T1.[ItemCode], T1.[Dscription], T1.[Quantity], T1.[Price]
FROM OWTR T0 INNER JOIN WTR1 T1 ON T0.DocEntry = T1.DocEntry
WHERE T0.[DocNum] >= '914141' AND T0.[DocNum] <= '914201'
INSERT INTO WTr1 (LineNum, DocEntry, ItemCode, Dscription, WhsCode, Quantity)
select MAX(T1.LineNum)+1, T1.DocEntry, '624262301882', 'Console 9.99', 'CONS', '25'
FROM OWTR T0 INNER JOIN WTR1 T1 ON T0.DocEntry = T1.DocEntry
WHERE T0.[DocNum] >= '914141' AND T0.[DocNum] <= '914201'
Group By T1.DocEntry
SELECT T0.[DocNum], T0.[DocDate], T0.[CardCode], T0.[CardName], T1.[ItemCode], T1.[Dscription], T1.[Quantity], T1.[Price]
FROM OWTR T0 INNER JOIN WTR1 T1 ON T0.DocEntry = T1.DocEntry
WHERE T0.[DocNum] >= '914141' AND T0.[DocNum] <= '914201'
INSERT INTO WTr1 (LineNum, DocEntry, ItemCode, Dscription, WhsCode, Quantity)
select MAX(T1.LineNum)+1, T1.DocEntry, '624262301899', 'Console 19.99', 'CONS', '15'
FROM OWTR T0 INNER JOIN WTR1 T1 ON T0.DocEntry = T1.DocEntry
WHERE T0.[DocNum] >= '914141' AND T0.[DocNum] <= '914201'
Group By T1.DocEntry
SELECT T0.[DocNum], T0.[DocDate], T0.[CardCode], T0.[CardName], T1.[ItemCode], T1.[Dscription], T1.[Quantity], T1.[Price]
FROM OWTR T0 INNER JOIN WTR1 T1 ON T0.DocEntry = T1.DocEntry
WHERE T0.[DocNum] >= '914141' AND T0.[DocNum] <= '914201'
INSERT INTO WTr1 (LineNum, DocEntry, ItemCode, Dscription, WhsCode, Quantity)
select MAX(T1.LineNum)+1, T1.DocEntry, '624262302056', '80 Stand-Up Displayer', 'CONS', '1'
FROM OWTR T0 INNER JOIN WTR1 T1 ON T0.DocEntry = T1.DocEntry
WHERE T0.[DocNum] >= '914141' AND T0.[DocNum] <= '914201'
Group By T1.DocEntry