Hi @CatherineLittle,

Unfortunately, as you've discovered, it's generally not possible to configure Turnitin after submissions have been made and have it go back and check the earlier submissions.  I think that's by design, so students all know and agree to their papers being checked by Turnitin.  The only way to have them checked within Canvas at this point would be to have the papers resubmitted now that Turnitin is configured properly.  I'd strongly suggest that students should do this themselves, but the teacher may be able to use the "submit on behalf of" feature if that's enabled in your Canvas instance.

-Chris