If you got this error when you entered your domain...
1. Your domain could be in an invalid form. Check your typing and spelling! Watch out for spaces and other unique characters ie http://.(As a side note, subdomains are usually sub.domain.com, and NOT www.sub.domain.com.)
2. You could be switching from www.domain.com to domain.com in the custom domain settings box. When you do this, the service we work through recognizes your domain as already taken. Instead, empty the box first, save/update it, and then enter the domain that you want!
3. You could have tried to connect this url to another Strikingly site. Check to make sure that you have emptied the setting on the other page.