I too have had this problem. I have used Canvas for a few year, but only with face-to-face courses. I am finding that Canvas is 100 % inadequate for running a course online...it fails at every second thing I try.
Students can upload, great. But try marking up a PDF, and the doc jumps all over the place, it was impossible to grade efficiently. I need to write on the tests to demonstrate the math properly...text comments don't do the same job.
Then, I could download all of the tests and graded them using Xodo (brilliant free app for pc). But tried uploading them exactly as the tutorial says, and I get the same error you do. I wonder if Blackboard users have as many issues?
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