Android studio design editor is unavailable until after a successful project sync

Question

what is the actual root cause of this issue? I already sync with gradle files but eventually failed.

Answer 1

I had this problem and it was because I didn't have the right API level installed (and I was also getting an error to that effect). I went into Tools --> SDK Manager and installed the API the system was telling me I needed (in my case, 30). I did that and restarted Android Studio and was able to get into XML files.