This is due to how Discourse makes unique URLs from thread titles, e.g. https://community.freetrade.io/t/title-has-already-been-used.
I can’t see any matching titles either, but that usually just means it’s in a hidden category or deleted.
Easiest way around it is probably just to change the title slightly, I think there is a Discourse setting but I don’t have access to alter it I’m afraid.