You should be using a join rather then individual AND conditions. try thi instead:

select MapRef, max(LstSlDate) as LstSlD1 into snap_new from snapshot
group by MapRef;

SELECT SnapShot.* INTO snap_new2
FROM snapshot INNER JOIN Snap_New ON (snapshot.LstSlDate = Snap_New.LstSlD1) AND (snapshot.Mapef = Snap_New.MapRef);