Hello @AnnaGrunwald ,
Thank you for posting on the Instructure Community!
The free for teacher canvas instance (domain), and a school district's canvas instance (domain) are two whole different sites with different logins, and there is no connection between them. That being said, the educators will be required to create a free for teacher account before being able to be added to your course, which is located on the free for teacher instance.
The reason this isn't working is because it's only looking for users under the free for teacher instance and not other school districts. Once those educators join Free for Teacher, you should be able to add them without any hassle. And if for some reason you're still unable to, provide them with access to a join code instead as a student, and edit their role to teacher after they join your course.
Hope this helps a bit!
Best,
Noah