You cannot put COALESCE around your sum. There are a few possible solutions, one of them might be:

SELECT
DM.ID_MAG AS NR_MAG,
EXTRACT(YEAR FROM DM.DATA_DOC) AS ROK, EXTRACT(MONTH FROM DM.DATA_DOC) AS MIESIAC ,
SUM(COALESCE(DMP.WART_NETT, 0) AS PRZYCH_MIES
FROM
DOC_MAG DM
LEFT JOIN DOC_MAG DMP
LEFT JOIN MAG_TYP_DOC MTD2
ON DMP.TYP_DOC=MTD2.ID
AND DMP.ID_MAG=DM.ID_MAG
AND MTD2.TYP=1
AND DMP.STATUS=1
AND DMP.DATA_DOC>=
CAST(EXTRACT(YEAR FROM DM.DATA_DOC) || '-' || EXTRACT(MONTH FROM DM.DATA_DOC) || '-01' AS DATE)
AND DMP.DATA_DOC<CAST(EXTRACT(YEAR FROM DATEADD(1 MONTH TO DM.DATA_DOC)) || '-' || EXTRACT(MONTH FROM DATEADD(1 MONTH TO DM.DATA_DOC)) || '-01' AS DATE)
AND EXTRACT(YEAR FROM DMP.DATA_DOC)=EXTRACT(YEAR FROM DM.DATA_DOC)
AND EXTRACT(MONTH FROM DMP.DATA_DOC)=EXTRACT(MONTH FROM DM.DATA_DOC)
WHERE
DM.ID_MAG=23
GROUP BY DM.ID_MAG, EXTRACT(YEAR FROM DM.DATA_DOC), EXTRACT(MONTH FROM DM.DATA_DOC)
ORDER BY DM.ID_MAG, 2 DESC, 3 DESC

HTH,
Set