Refusal to get refund by agency

I accidentally booked a one way ticket to Australia via flight centre. As a visitor I cannot board without a return ticket, I’ve been trying to get a refund or at least upgrade my flight but I’m being told that I cannot get a refund and when I ask for an upgrade I have to buy a return flight instead of topping up my one way. I need someone who’ll help with reading the terms of my flight because there’s no where it says I can’t get a refund