Unable to Access an Error Message Corresponding to Your Field Name

[eluser]01010011[/eluser]
Hi,
I've created this registration form (see attached) that registers new users and checks whether their user name already exists in the database.

The problem is, instead of displaying the error message I wrote, it displays this error message:

Unable to access an error message corresponding to your field name

How do I correct this? Any assistance will be appreciated. Thanks in advance.