@JDK
I don't know about the validity of this statement "I know new quizzes is enabled because I have many in my course."
I have a sandbox course where I played with new quizzes a few years ago. At that point, it wasn't available in beta, so I had to enable it for the entire school to play with it in my sandbox. I would go in, create a new quiz to test something for people in the Community, and then disable it for the institution. The new quiz remained in the course, though, even though new quizzes wasn't available. At some point, new quizzes got messed up in that course and I can not longer use them at all.
I have another sandbox course that I made to fix the issue with new quizzes. I went into it just now and the option to reset the engine is still there. You might go into your course settings and choose features. Verify that new quizzes is enabled there. It might also be that your local Canvas Administrator has disabled new quizzes globally without realizing that people were using it.
There is a way to reset the engine using the Canvas REST API, but likely won't have any effect if new quizzes isn't enabled so verify the settings first. I didn't see it documented where I thought it would be, but I looked at the network traffic that Canvas sends when you choose the reset option to find it. You can use the PUT method to send newquizzes_engine_selected=null to <canvasInstance>/api/v1/courses/1234/quizzes (where 1234 is the Canvas course ID). Using the API directly isn't trivial so I would check the settings first.
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