I think you're trying to access a custom object as if it were a standard marketo lead field. You can't!
you need to create a script (velocity) token to read in abandoned cart object(s). The (s) is important, as there can be more than one abandoned cart here.
You'd sort the list of abandoned carts into date order, get the most recent, and then output whatever details you want into the email, all within the Velocity Script.
You also need to make sure the cart is still abandoned. Is Magento only sending these to you at a pre-determined time (i.e. when it decides it is abandoned), or does it flag the cart as completed if someone completes the purchase?