Hello Melissa,
Often, instructors can't add people to their classes after the semester is over and the end date of the course has passed. However, that depends on the settings your school uses, so you'll want to ask the Help Desk or Canvas Support at your school.
There are other ways to share your course with another instructor without adding them to the course, though.
You can export and save the course to a Canvas export file, and share that file with the other instructor. There are instructions at https://community.canvaslms.com/docs/DOC-12785-415241323
Mike
