Unable to create new mail account

Are you just trying to create a new mailbox, or are you trying to create a new account? Those are two different things. This is how to create a mail account on the iPhone. Add an email account to your iPhone, iPad, or iPod touch - Apple Support Use this to create a new mailbox folder in an existing account. Use mailboxes to organize email on your iPhone, iPad, or iPod touch - Apple Support

When you say create, do you really mean that you are just trying to add the account to your mail app? If so, what happens when you attempt to add the account to the mail app on your iPhone? Do you see an error message? If so, what is the exact text of the error?