When I couldn’t open my trial account at first, I just asked a very good friend to open a trial account for me with his card. Since we trust each other and I’ll only use free features, it was not a problem.
But it could be interesting to know what you intend to do with your forum. Build a community? Then, don’t go for the free Oracle plan. It’s very, very slow. You can’t avoid a paid plan for this kind of purpose.
Install Discourse just to try stuff as an admin by yourself and do experiment? Then install it on your local machine, it won’t cost a cent.