This is typically a result of the Friendly URL already being used by another participant or team.

Another reason this could occur is in this scenario:

A participant attempts to register and enters their friendly URL at the time of registration. The registration fails for various reasons. It is possible, even though the registration failed, that the Friendly URL is unavailable.

A support analyst can attempt to release the friendly url for you (BB710573).  Chat with Support and reference this article.  Also include the event's name, url, page in question's raw url and the friendly url.