The bottom line is this: until you publish your course, you cannot send out announcements or Inbox messages. You must publish the course first.
There are a couple of options for you, and I do both:
- Go into your school's SIS system and send out a mass email to your students. (I do this a week before the start of the new semester.)
- Publish your Canvas course ahead of time. I suggest having a welcome message on your Home Page and posting your Syllabus. Keep everything else in your course (all of the Modules) unpublished. If you do that, you can send out an Announcement as well. (I also publish my course shell a week early.)
Of the two methods, a mass email is the better option for reaching your students. If your students are new to your school, they may not be familiar with Canvas. Also, your students may not know how to have Canvas Announcements linked to their email accounts.
Please come back and let me know if this information is helpful to you. I'm just a fellow instructor volunteering my time. 😀