How can I fix “Design editor is unavailable until a successful build” error?

Question

I started to learn Android Studio from yesterday. I try to echo out simple \"Hello World\":

Answer 1

In my case, the Windows Firewall was blocking Android Studio (i.e. java.exe from Android Studio's jre/bin folder) from connecting to sites and downloading data. After Windows asked me if I wanted to allow the app through the firewall, I had to do a manual sync File -> Sync project with gradle files. After that, things were still disfunctional, a manual download of an SDK version 28 was necessary from Tools -> SDK Manager.