Depending on the username you are trying to rename to, it may already be in use or be too similar to another that is already in use. See Special:ListUsers and try searching your username in the "Find by a username" box. If your desired username is not in use, try leaving off the last few characters to see if other similar usernames show up.
If the latter is the case, staff will often consider rename requests which are unique, but disallowed by AntiSpoof for sharing too many characters. Contact staff by creating a new support ticket.