Currently the Readarr metadata server is down. This will prevent you from doing searches or adding books and authors. There is no timeline for getting it back up, but the devs are working on it. The metadata server is not open source, and there is nothing you can do to help get it back up.
You can add SOME authors by finding the author ID number on goodreads (it's part of the URL) and adding it to readarr by using author:authorid in the search.