I just click on Communities and then click unsubscribe next to the one I want to drop.

I'm using connect app on android. What are you using?

If you can't unsubscribe, I suppose you could block community?

Is it possible you mean how to unsubscribe from a server/instance? Because that is impossible for individual users at the moment

If you use jerboa, click the joined button to unsubscribe the community. Different apps may vary.

If you uses lemmy-ui, see comment below.

If you cannot unsubscribe at the moment, you may facing a server error, try again after few minutes.