403 error on user registration

I’m attempting to register a new user using Postman..

/api/user/register/?username=john&[email protected]&nonce=be66a73631&display_name=John

After generating the nonce, I send the above POST, and the returning value in the console is:
{“status”:”error”,”msg”:”account duplication”}

I change values to be unique and it is the same error. Though I note a 403 status is returned.

any one else have this issue or can point me in the right direction?