I am not 100% sure, but I think that is automatic when you sub to a remote community.

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

It’s a bug, I have been able to tap it and hit Subscribe again and it works after a couple seconds.

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.