Subquery in MS Query doesn't work
Your query can be simplified (regardless of SQL flavor) not to use an inner query (subselect). This is not tested, but it should be closer to what you need:

SELECT tPLAN.Jahr, tPLAN.Monat, tPLAN.Land_ID, tPLAN.Kanal, T1.tPLAN, tPLAN.Planstand, 
    tZ.Zeitraum, T2.IST_ITEMS_PER_HC_EXCL_EASY 
FROM t_Plan_Items_per_HC tPLAN 
    JOIN t_PLAN_ZEITRAUM tZ ON tZ.Jahr = tPLAN.Jahr AND tZ.Monat = tPLAN.Monat
    JOIN a_ITEMS_PER_CALL_ACT_MONTH_KANAL_EXCL_EASY_HC AS T2 
        ON (T2.Zeitraum=tZ.Zeitraum AND T2.Land_ID=tPLAN.LAND_ID AND T2.Kanal=tPLAN.KANAL) 


Since all JOINs are INNER, you can just select from one of the tables and JOIN with each of the others using your criteria. Also, WHERE conditions like:

FROM t_Plan_Items_per_HC tPLAN, t_PLAN_ZEITRAUM tZ
    WHERE tPLAN.Jahr = tZ.Jahr 
    AND tPLAN.Monat = tZ.Monat) 


are not recommended. Use JOINs instead.