There is no '+ Rubric' button for my assignment. According to the guide, this button should exist, but it's not there.
Hi Michelle Henninger , and Welcome to the Canvas Community!
In a hurry, but this guide will help How do I add a rubric to an assignment?
Bottom line - you must be in edit mode.
Hi, Michelle, clearly Kelley was in a hurry : I think he meant to say "you must not be in edit mode." Also, you won't see the Add Rubric button if the assignment is of the External Tool type. The way to get around that is to start out by creating an online submission type assignment, saving it, adding the rubric, and finally re-editing the assignment settings to switch the assignment to the External Tool submission type.
If my guess is wrong and the assignment is not of the External Tool type, you might want to submit a help ticket to have Canvas Support take a look (How do I get help with Canvas as an instructor?)
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