It looks like you are trying to use the dlookup to pull data from a diferent table other than the one you have on your query. That would not work. Dlookup only works on the current dataset. Here is how you should rewrite your query:
SELECT
InventoryTransactions.ProductCode,
Manufacturer,
InventoryTransactions.TransactionID,
InventoryTransactions.TransactionDate,
InventoryTransactions.PurchaseOrderNo,
InventoryTransactions.TransactionDescription,
InventoryTransactions.UnitPrice,
InventoryTransactions.UnitsOrdered,
InventoryTransactions.UnitsReceived,
InventoryTransactions.UnitsSold,
InventoryTransactions.UnitsShrinkage
FROM
InventoryTransactions
inner join Products1 on [inventoryTransactions].[ProductCode]=[Products1].[ProductCode];
Hope this helps.