From the discussion below,
It seems that Access executes the base query first, and then apply the dynamic ‘where’ condition, and thus you can only filter by columns included in your select list.
SELECT B.Owner, A.Fundno,Sum(B.AvLand) AS SumOfAvLand, Sum(B.AvImpv) AS SumOfAvImpv, Sum(A.TaxAmt1) AS SumOfTaxAmt1, Sum(A.TaxAmt2) AS SumOfTaxAmt2, Sum(A.Unpaid1) AS SumOfUnpaid1, Count(A.Napn) AS CountOfNapn
FROM [Solano Delinquency 2009] AS A INNER JOIN [Solano Roll 2008] AS B
GROUP BY B.Owner,A.Fundno;