You don’t. The server just can only handle a finite amount of actions per refresh. If you wait like 10-15 seconds your subscribe will go through. It’s got nothing to do with approvals.
There isn't an approval process to subscribe to communities - anyone with a Lemmy account can subscribe. What are you seeing that looks like you need approval?
There can be an approval process for signing up to an instance/server, though.
When I search for a community that isn't on my local instance, I often get a "subscription pending" text, instead of it immediately changing to "joined".
I don't think it's about approval (there is no "approve subscribers" option when setting up new communities on lemmy). I think it's a bug or burp in the process. I had about 5 pending the other day and now only one shows it now. All of them function normally.