I sympathize with your problem, I also couldn't find it. Not seeing the accessibility checker icon may have to do with using a smaller monitor or laptop screen.
The solution in my case was to press the Ctrl key and the minus key, so you can see more of the screen (or you can sometimes use your arrow keys to tab through the icons and find the accessibility checker button that way).
Evidently Instructure, in their wisdom, did not foresee that not everybody has a widescreen monitor.
regards, P.
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