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 is listed, that it's turned "on" and that the ID is 170000000000514. (Screen shot below).
If the key already is enabled, try toggling it off, then back on to see if that helps.
If you do not see this listing at all, try expanding the listing by clicking on the "show all keys" button at the bottom of the page:
Be sure you are looking for "Respondus LockDown Browser," not "Monitor," or "Respondus Monitor." If you still cannot find the necessary key, please contact Instructure for assistance in setting up the key for your instance.