The following are possible:
- Another user has already registered the username.
- Contains invalid characters.
- The number of characters is too few or too many.
The characters and the number of characters that can be used are as follows.
- Lowercase (a-z) and uppercase (A-Z) English alphabet characters, numbers (0~9), underscores (_), and periods (.)
- Minimum of 8 characters and maximum of 14 characters