Hola compañero, como dice @jose_saul_san debes de tener las mismas columnas en las dos consultas, mi consejo es que, primero corras tus consultas individuales para checar errores ya que esten corriendo al 100 las unes
comentarie tu concatenacion por que SAP B1 sql server no corre el LPAD lo use en ORACLE alguna vez.
SELECT
T1.DocNum AS 'Nro Recibo',
'Fact. D.' as Documento,
T1.Canceled,
T1.TaxDate,
T1.DocDueDate,
T0.CardCode,
T1.CardName,
T1.DocTotal AS 'Tot s/ret',
T1.U_Total_Documento,
--(T0.Letter || LPAD (T0.PTICode, 4, 0) || '-' || LPAD (T0.FolNumFrom, 8, 0)) as 'Nro Comprobante',
T0.DocCur AS Moneda,
T0.DocTotalSy AS Total,
T0.VatSum AS IVA,
T0.DiscSum as Redondeo,
T0.WTSum as Retención,
T0.U_UN,
T0.U_CC
FROM OINV T0
INNER JOIN ORCT T1 ON T0.ReceiptNum = T1.DocEntry
WHERE T1.TaxDate >=[%0] AND T1.TaxDate <=[%1] AND T1.JrnlMemo NOT Like '%%Cancelado%%'
UNION
SELECT
T1.DocNum AS 'Nro Recibo',
'Notas C.' as Documento,
T1.Canceled,
T1.TaxDate,
T1.DocDueDate,
T0.CardCode,
T1.CardName,
T1.DocTotal AS 'Tot s/ret',
T1.U_Total_Documento,
--(T0.Letter || LPAD (T0.PTICode, 4, 0) || '-' || LPAD (T0.FolNumFrom, 8, 0)) as Nro Comprobante, teca tu concatenacion
T0.DocCur AS Moneda,
T0.DocTotalSy AS Total,
T0.VatSum AS IVA,
T0.DiscSum as Redondeo,
T0.WTSum as Retención,
T0.U_UN,
T0.U_CC
FROM ORIN T0
INNER JOIN ORCT T1 ON T0.ReceiptNum = T1.DocEntry
WHERE T1.TaxDate >=[%0] AND T1.TaxDate <=[%1] AND T1.JrnlMemo NOT Like '%%Cancelado%%'