Hey, it sounds like the app you're trying to use expects the server to be running Lemmy 0.18.0, which is the latest version.
I'm sure lemmy.world will upgrade in a couple of days, I believe they are waiting for 0.18.1 to release. In the meantime, you can try making an account on another instance that is already running 0.18.0 and using that to browse/interact with the Fediverse through the app.
You're welcome to create one on lemdit.com if you wish.