I am trying to execute this trade: https://dispenser.tf/trade/86465/
However, every time I consistently run into the below error:
In the network tab of DevTools I can see the request to backend fails with 500 error. Other trades work, but this one always fails with this 500 error.
https://dispenser.tf/trade_offer_purchase/
Server Error (500)

It's an old bug for this less tested code path, should be fixed now.

This trade doesn't work because it allows flexible TF2 metal, which an item selection dialog will be presented to you to fulfill the 3 refined metal with reclaimed or scrap metal in your inventory.