Hi @bludwig
Depending on your course setting (visibility) your added observers are likely getting that message because they do not yet have a user account in your Canvas instance.
Don't change your settings - talk with your school's Canvas Admin/Support folks first. They can enable the Observer functionality if it isn't already enable, they can then develop a process for adding observer-users, and there may be policies and procedures governing enrollments (even as an observer) at your school. And there will likely be policies around the "visibility" state of your courses in Canvas.
I know this isn't much help, but doing it right will work out better in the end.
Kelley
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