Hello! First Post.
I recently made my OWA booking and then added a few flights a week later, I got the exact same issue when I called up to add those flights. There was a currency issue between CNY and AUD, my OWA starts in Shanghai.
With the first Lady I talked to she had to put me on a hold for a bit and told me she'd call back, surprise surprise there was no call back but she had saved my flights so I didn't lose the availability (shows up on the Qantas app) . So I called the next day and got another lady, same issue again, got put on hold and told me she would call me back. This time I did get the call back and she was able to charge my taxes, however it was in CNY which I was fine with because I had an international card.
Like others suggested HUACA, but this currency issue seems to be a common problem that needs escalation from the call centre. For now, just get those additional flights 'saved' so you don't lose the availability, and keep calling, not sure how long it stays saved.