Thanks for posting. Sorry to hear you are having an issue with creating a clan. When creating clan and choosing a name results in it's already in use, you may have to try a more unique clan name. We'll look into the issue as to why the name does not appear in your search results. If you like, you can shoot me a DM of the clan name and we'll check.
Also, what is the specific error message you are receiving when creating a clan. Are you using the console app when you receive the error message?