Why can't I send or accept trade requests?

Why can't I send or accept trade requests? For example, I messaged a player saying I wanted to buy his ring. He invited me to a party, and once we were in the party, whenever he sent me a trade request, it was automatically declined. I don't even have that option in my menu.
I tried trading with other people, and the same thing happened. They send me a request — I automatically decline it (the chat says 'Trade cancelled'). I just can't initiate a trade either