I only have a basic understanding of how any of this works, but I saw someone else having a similar problem earlier and it had to do with their server being defederated from the server hosting the community they were trying to subscribe to, I think?
There’s definitely something weird going on between those two instances. I just popped over to my Lemmy.world account and couldn’t find that community in the exact same search I just did on this account. Maybe something having to do with Lemmy.world getting hacked yesterday and someone on Lemmy.ml still being overly cautious?
Edit: and just now I tried again and got different search results, found the sub, and was able to subscribe. Weird. Glitch in the matrix?
You can subscribe to both the lemmy.world and lemmy.ml communities. Neither seem as established as the subreddit anyways (I would know, that subreddit is still number two in my Firefox recommendations even though I haven't been there in weeks...)
As mentioned in this thread, you can just use the regular search to find both communities. If one of use subscribes to a community on a different instance, you should be able to search for it. Otherwise, use a web browser to subscribe to it.