wp user frontend multiple forms

Hey,

I’m using the WP User Frontend plugin but I wanted to use multiple users.
So, I downloaded this: https://reigelgallarde.me/programming/wp-user-frontend-multiple-forms/
But when I try to make a form and fill in a frefix I get this error: Please use another prefix. Prefix already present in.
I’m getting this error with every prefix !
How can I fix this?
Many thanks in advance,

Thibault