A few things. In your table, you have your Amount field formatted as Text. You should choose a Numerical option instead of Text.
I am not sure why the NZ function isn't working when there is nothing in the SubReport, but perhaps we can go about this another way. We can make a query shows the records and does the calculations, and then use that as the basis of our Report (no subreport).
Based on your example and sample queries, here is what that code may look like:
Code:
SELECT
Query1.[Dr name],
Query1.[Systems posted on],
Query1.Amount, Query2.[Systems posted on],
Query2.Amount,
[Query1]![Amount]+nz([Query2]![Amount],0) AS Total_Amount
FROM
Query1
LEFT JOIN
Query2 ON Query1.[Dr name] = Query2.[Dr name];