This might be obvious, but it's a common mistake made by teachers new to Manhattan. You can't post a message to say, the Handouts/Notices module, that includes a written assignment with the instructions "Submit your work via the Assignments module" unless you post an assignment in the module for them to reply to.
When students enter the Assignments module for a new Manhattan classroom, they can't post any messages. They don't have a New Item button! Students can only post messages that are a reply to an existing message or assignment.
If your course syllabus, posted to the Handouts/Notices module, includes details for each assignment in the course, you don't have to duplicate all that work in the Assignments module. Instead post a new Assignment, for each detailed assignment in your syllabus, that says in effect "Post your answers to: ____ here!" or "Post your final paper here!"