Due to the decentralized nature of Forex trading there is no Last Sale reported. The TWS Time and Sales window for an FX pair will not have a Last Sale column, but rather will display the Midpoint of that FX pair's current Bid/Ask. If you are looking to see why an order did or did not fill, you can use the Time and Sales window to review an FX pair's Bid/Ask data around the time you believe your order should have been filled.
In addition, there is no "Last Sale" choice when electing a Stop Trigger Method for a Forex order. The only Stop Trigger Methods available are Default (Single Bid/Ask), Double Bid/Ask or Midpoint.
For more information, please visit the TWS Users' Guide. If you need assistance with a particular order, please contact the IBKR Trade Group and provide the order ID number or a complete description of the order and the exact time from the TWS Time and Sales that you are referencing.