When I open a new course downloaded from iTunes U, the app pops up a dialogue box asking for an Enrollment Code. Where do I get an enrollment code?

I don't know where to get this Enrollment Code. When I try to log into my iTunes account with my Apple ID, it tells me the Enrollment Code I entered (did not enter anything) is invalid. I don't know if this is something that is required for iTunes U or not. I can view and navigate through courses, but will this keep me from receiving credit for completion? Any insight would help.