Unable to use subquery into group by

I get this error:

Msg 144, Level 15, State 1, Line 68
Cannot use an aggregate or a subquery in an expression used for the group by list of a GROUP BY clause.

when running this code:

SELECT 
    CASE 
        WHEN O.Id IS NULL
            THEN 0
            ELSE O.Id
    END AS TabellaOrdiniId,
    OM.ID AS IdOrdineMexal,
    OM.csg_doc + RIGHT('00' + CONVERT(VARCHAR(2), OM.NGB_SR_DOC), 2) + '-' + RIGHT('000000' + CONVERT(VARCHAR(6), OM.NGL_DOC), 6) AS OrdineMadre,
    CASE 
        WHEN O.Id IS NULL
            THEN (  -- >>>look here: *** <<<
                  SELECT TOP 1 [dbo].[vAP_StatiOrdine].Id
                  FROM [dbo].[vAP_StatiOrdine]
                  WHERE [dbo].[vAP_StatiOrdine].Attivo = 1
                    AND [dbo].[vAP_StatiOrdine].Id = CONVERT(INT, ISNULL((SELECT TOP 1 [dbo].[tParametri].[ValoreParametro]
                                    FROM [dbo].[tParametri]
                                    WHERE [dbo].[tParametri].ChiaveParametro = 'IdStatoOrdineDiDefaultOrdini'), 0))
                    )
        ELSE O.IdStatoOrdine
    END AS IdStatoOrdine,
    SUM(OD.[PrezzoDiVenditaTotaleNonIvato]) AS TotalePrezzoDiVenditaNonIvatoOrdine
FROM 
    dgw_rp..DGW_ORDC_M AS OM
LEFT JOIN 
    dgw_rp..DGW_RUDT AS C ON C.CKY_CNT = OM.CKY_CNT_CLFR
LEFT JOIN 
    dgw_rp..dgw_ISPED AS ISPED ON ISPED.CKY_INDIR_ALT = OM.CKY_SPED_COD
LEFT JOIN 
    tOrdini AS O ON O.IdOrdineMexal = OM.ID
LEFT JOIN 
    tStatiOrdine AS SO ON SO.Id = O.IdStatoOrdine
LEFT JOIN 
    tTipiIncasso AS TI ON TI.Id = O.IdTipoIncasso
LEFT JOIN 
    tTipiServiziCorrieri AS TS ON TS.Id = O.IdTipoServizio
LEFT JOIN 
    tTipiBolla AS TB ON TB.Id = O.IdTipoBolla
LEFT JOIN 
    vAP_OrdiniDettagli AS OD ON OD.IdOrdineMexal = OM.ID
LEFT JOIN 
    tAnomalieOrdiniByCorriere AS AOC ON AOC.IdTabellaOrdini = O.Id
GROUP BY 
    CASE 
        WHEN O.Id IS NULL
            THEN 0
            ELSE O.Id
    END, OM.ID,
    OM.csg_doc + RIGHT('00' + CONVERT(VARCHAR(2), OM.NGB_SR_DOC), 2) + '-' + RIGHT('000000' + CONVERT(VARCHAR(6), OM.NGL_DOC), 6),
    CASE 
        WHEN O.Id IS NULL
            THEN (
                    -- >>>look here: *** <<<
                    SELECT TOP 1 [dbo].[vAP_StatiOrdine].Id
                    FROM [dbo].[vAP_StatiOrdine]
                    WHERE [dbo].[vAP_StatiOrdine].Attivo = 1
                        AND [dbo].[vAP_StatiOrdine].Id = CONVERT(INT, isnull((
                                    SELECT TOP 1 [dbo].[tParametri].[ValoreParametro]
                                    FROM [dbo].[tParametri]
                                    WHERE [dbo].[tParametri].ChiaveParametro = 'IdStatoOrdineDiDefaultOrdini'
                                    ), 0))
                    )
        ELSE O.IdStatoOrdine
    END

The problem seems to be the subquery. Also in field set and group by clause.

But I don't understand why: maybe because the query engine can't recognize TOP 1 and "believes" that the subquery returns more than one value.

If I replace the subquery with a scalar function, or an integer constant, everything works.

Can anyone explain me why I can't use subquery in group by clause?