This will not work.
You need to add the dUserFK in the SELECT of your left join...
It is not because you select 1 field that SQL knows what all the other fields of that table are.
So try this :
SELECT Q1.uUserName, Sum(IIf([Q2].[dCatFK]=1,1,0)) AS AM, Sum(IIf([Q2].[dCatFK]=2,1,0)) AS PM
FROM...