It sounds like their account information is inaccurate. I would suggest reaching out to your Canvas admin to verify that the correct email addresses have been used.
In the meantime, since it sounds like you are in contact with the students, you could use a workaround by enabling self-enrollment for your course and giving the students the join code or secret URL. You can learn how to do that via the help guide below:
How do I enable course self-enrollment with a join code or secret URL?
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