MongoDB : "Couldn't add user : Could not find role"

I'm trying to create a role and an user for my MongoDB database. I created the role, but when I try to create the user I get this error : "Couldn't add user : Could not find role XXX ". Ok ... But when I try to create the role again, mongo tells me the role already exists. What am I suppose to do please ? It's a school project, I'm not familiar with MongoDB.

MongoDB errors