Cannot SUM the value of a sales amount field - various criteria, sales rep id and date ranges
There are more compact ways to write this, but this illustrates a way to do it.
First select the territories and periods that Anne was looking after that territory.
Then pull all the sales where the transaction date falls in one of the periods when Anne was covering the territory where the sale was made.
Then sum up the sales.
WITH
 territory_periods AS (
SELECT SalesRep, TerritoryID, FromDate, ToDate
  FROM tblSalesRep 
 WHERE SalesRep = 'Anne'
)
,sales AS (
SELECT p.SalesRep, s.Amount
  FROM tblSales s
  JOIN territory_periods p
    ON s.TerritoryID = p.TerritoryID 
   AND s.TransactionDate BETWEEN p.FromDate AND p.ToDate
)
SELECT SalesRep, Sum(Amount)
  FROM sales 
GROUP
    BY SalesRep
;