I found this information that might help. I too would like to know how to post a discussion.
"Yes, Canvas includes a discussion feature, and these can be found in the Discussions content area, or embedded in their appropriate modules depending on how the instructor has set them up. Many faculty who organize their classrooms using Modules, will hide the Discussion content area (along with other content areas) to avoid confusing students. You can learn more about Discussions in the Discussions Chapter of the Student Guide.
If you are a student, and are looking for the Discussions in your course, but don't see the Discussions content area in your course navigation menu, then check modules. If you don't find the discussions posted there, it is likely that your teacher does not use them. I would ask your teacher.
If you are a teacher, and want to learn how to create and use discussions, you can learn more in the Discussions Chapter of the Instructor Guide."
If you go to: https://community.canvaslms.com/community/answers , there's a red button on the left hand side that says "Start a Discussion". This might be what you are looking for.
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