Use a calculated column in a where clause

Using Derived Columns in a predicate

You'll need to wrap the inner query in a derived table or CTE in order to be able to use derived columns in the WHERE clause (Also, note SUM() is specified just once, using the results of the multiplication):

SELECT x.Code, x.AccountNumber, x.Sales
FROM
(
  SELECT p.Code, c.AccountNumber, SUM(p.UnitPrice *od.QtyShipped) AS Sales 
  FROM [dbo].Customer c 
      LEFT JOIN [dbo].OrderHeader oh ON oh.CustomerId = c.Id 
      LEFT JOIN [dbo].OrderDetail od ON od.OrderHeaderId = oh.Id
      LEFT JOIN [dbo].Product p ON p.Id = od.ProductId
  GROUP BY p.Code, c.AccountNumber
) AS x
WHERE x.Sales > 100;

Repeating the Derived Column in a HAVING clause

As per @Jonny's comment, the other way is not to DRY up the calculated column, but to instead repeat the calculation. Use HAVING instead of WHERE after a GROUP BY has been applied.

SELECT p.Code, c.AccountNumber, SUM(p.UnitPrice *od.QtyShipped) AS Sales 
FROM [dbo].Customer c 
  LEFT JOIN [dbo].OrderHeader oh ON oh.CustomerId = c.Id 
  LEFT JOIN [dbo].OrderDetail od ON od.OrderHeaderId = oh.Id
  LEFT JOIN [dbo].Product p ON p.Id = od.ProductId
GROUP BY p.Code, c.AccountNumber
HAVING SUM(p.UnitPrice * od.QtyShipped) > 100;

In either case, as per comments below, note that the calculated expression is SUM(p.UnitPrice * od.QtyShipped) and not SUM(p.UnitPrice) * SUM(od.QtyShipped).

You can use the common table expression for this

;WITH CTE AS
    (
    SELECT p.Code, c.AccountNumber, Sales = (SUM(p.UnitPrice) * SUM(od.QtyShipped)) FROM [dbo].Customer c 
        LEFT JOIN [dbo].OrderHeader oh ON oh.CustomerId = c.Id 
        LEFT JOIN [dbo].OrderDetail od ON od.OrderHeaderId = oh.Id
        LEFT JOIN [dbo].Product p ON p.Id = od.ProductId
    
    GROUP BY p.Code, c.AccountNumber, Sale
    
    )
    
SELECT *
FROM CTE WHERE CTE.Sales>100