Assignment

Several possibilities.

1. You've changed student capabilities to prevent/prohibit?
2. More likely, the assignment is not in the current classroom but in a template or another classroom. To check this, compare the URL the student is using with the URL of the actual assignment.
3. The assignment is hidden (eye closed).
4. There's a date to start and finish and students are outside of those dates.

This post dates back to 2010, but something that I experienced I would like to share here. When you get an error like this, it is likely that at the time of "switching role" from Teacher to "Non-editing teacher" or "Student" you won't longer have the privilege to see that particular page on which you were at. For example, you may be looking at enrolled users table and you suddenly switch your role then you get this error. But, if you notice your role under your profile image at the right corner of the screen, you can see your role has been switched to what you wanted.

You can navigate back to view the course by clicking the link title in the breadcrumb or go to My courses.

If  you are in a less restricted page like Course detail view. Switching roles should be smooth.

This indicates that the role permissions on a Moodle installation may not be incorrect unless it had been altered previously. I found that before diving deeper into inspecting permission configuration it is a good idea to understand the common logic behind why the system threw that error to me, and I think back the steps I performed to see if I can regenerate the same error again since a large number of developers' error reports are very generalized to cover unforeseen circumstances.