It means that 'Mora' has already been taken by someone. However, it doesn't mean that if there's none with Mora as display name, so none has it. There's either someone from this list uses it so or a banned forum account does. Pay attention for the error message, it tells you not only display name, but username too.
as Faith said the problem is not only in the display but it's on username too, so you can use a number instead of the comma like "Mora3" or "M0ra" ...etc.