It sounds like you're looking for a way to use Turnitin independently of your courses to check the similarity of your Comprehensive Exam paper. While direct access to Turnitin might be restricted to certain courses or through specific faculty members, there is an alternative you can consider: Scribbr.
Scribbr offers a plagiarism checker that's powered by the same technology as Turnitin. This means it uses a comprehensive database to check for similarities in your work, offering a level of scrutiny similar to what you'd get from Turnitin. The advantage here is that Scribbr's basic service is free to use, which should be sufficient for your initial checks. This free report provides basic insights into similarity and potential plagiarism in your document.
If you find that you need more detailed reports or advanced features, Scribbr also offers paid options. These can provide a more in-depth analysis, similar to what you would expect from a full Turnitin report.
Using Scribbr could be a convenient and effective way for you to independently check your work for plagiarism, without having to navigate the limitations of your school's Turnitin access through Canvas. You can simply upload your document to Scribbr’s platform and receive a similarity report.
This should help you in ensuring the academic integrity of your Comprehensive Exam paper without having to rely on the specific course settings for Turnitin access at your school.