Broker question

Using a demo account at Forex dot com I have an open AUD/USD position I have been trying to close, and I under the impression I might be able to close it with USD/AUD position.
So I tried that and got no ware.
The Question is; do Brokers allow traders to decide what currencies to trade?

Did you place a buy or sell order? For a buy order don’t you just click the X next to your green market entry price, and if a sell, the X next to the red sell entry price, to close them?

You can trade whatever currencies a broker supports. All of them should support at least the majors. And then some exotics mixed in.