This can be one of a few issues:
1. The profanity filter for Hero Academy is very strict. For most devices it will ask you to choose a different user name. For Android devices, it may display a more general error instead.
2. There is an issue with the @ entry and Hero Academy on some Android devices.To workaround this (we are working on a fix), in a note-taking application, type out your email address. Then copy it to your clipboard. Open Hero Academy and paste your full email address in the email box. It should properly read your desired login. (If your password contains an @ you will need to do this for the password as well.)
3. A solid connection with the server is required to register a new account. If the issue isn't one of the above issues, you can troubleshoot your connection.