If students are having trouble joining your course — for example, reporting that they need a passcode or that the enrollment link you distributed isn't working — the most common issue is that the link you distributed got modified and doesn't include the "enroll" parameter. (This can happen when sending email through some learning mangement systems; the linked text may still show the ?enroll=abcdefg portion of the link, but it isn't part of it.)

On the Students tab of your course page, you can click "Add/Remove Students" to get the enrollment URL (which will change if you modify the course passcode.) Double-check that the link you distributed matches this URL.

If that's not the problem, get in touch with Wiki Education staff via the Get Help button.