First, check your Role within the Canvas Course:

1. Open your Canvas Course, and click on People on the left
2. Verify that your Role is 'Teacher' on the list

If that doesn't resolve the issue, check that Flipgrid has automatically generated the Topic for your Assignment. If not, you can trigger the process manually:

1. Open the Assignment where you added Flipgrid as the External Tool
2. Click Load [Assignment name] in a new window, if that button is showing
3. You should be able to see your Flipgrid Topic now
4. Go back to Home in the Canvas Course
5. Click Student View on the right
6. Navigate to the Assignment again, and open Flipgrid