Students have not received the course invitation

A. There could be three reasons for this:

Course invitations will not be sent until after the course start date. The start date is commonly the term date, unless the term is being overridden by specific dates in Settings. In the latter case, you must tick the box under the new dates 'Students may only participate...'
Course invitations are not sent to users who are added via automatic data provisioning processes – such as using the Group Enrolment Tool (GET).
You must publish a course before students can access it and its contents. Students cannot see unpublished courses and content.

If some individuals still do not receive the invitation, you can Resend all of some of the invitations from the People Page.