If you are receiving the above error when either opening the LockDown Browser Dashboard LTI tool, or running the integration test for your Respondus Server profile, you should first check to see if the relevant developer key is installed and enabled.
To do this, in your Canvas instance log on with admin level credentials, then navigate to: Admin > Developer Keys > Inherited
Make sure Respondus is listed, that it's turned "on" and that the ID is 170000000000427. (Screen shot below). If you do not see this listing at all, contact Instructure for assistance in setting up the key for your instance.
