Seems to be a bug, I think it happens most frequently when trying to subscribe to a community on a busy instance. It doesn’t seem to affect functionality though, I see new posts, vote and comment, etc. Sometimes if you refresh the page or try again it goes away and says subscribed, but I wouldn’t worry, doesn’t seem to change anything right now. Just assume it means subscribed. I think for this community right now it says “sub pending” for me actually.

probably the creator of the community want to select who can enter and who cannot, it’s like for private Instagram profile, the owner have to accept your request

as @Wander@yiffit.net said:

the instance is probably overloaded or the instance of that community is defederated from your’s