Changing Permalinks

I wish to simplify and clarify the web page addresses for my site – for example from http://robbiedrewesart.com/?page_id=671 to http://robbiedrewesart.com/. I went to Settings -> Permalinks -> Custom Structure. I typed in http://robbiedrewesart.com/ in the box and Saved Changes. It immediately went back to the default permalink setting and left the Custom Structure box blank.

What am I doing wrong? I tried to follow the examples of the other settings. Is here a special syntax for setting the permalinks?