Are you actually looking at it from your instance? The post should say via myinstance.com at the top. I think there is someone working on making this behavior more intuitive. Also I posted this on liftoff.

I've got the same problem. In order to subscribe to communities from different instances I need to do it through a browser first. Only then I'm able to post using liftoff.

we're in the wild west period of Lemmy app development it seems haha. all we can do is hang in there