Vittore if you are self-hosting, you can follow these instructions to gather more information that will be useful for your support request https://docs.flarum.org/troubleshoot
If you are using a managed Flarum hosting like FreeFlarum you should contact their support directly.
In either case, do you have the markdown and/or bbcode extensions enabled? Have you tried bbcode to see if it works? Does any Markdown work at all? The output of the php flarum info command as suggested by luceos will tell us which extensions are enabled and which version you are running.
