I am creating an assignment on Canvas, and the little "+" sign with "add rubric" is not present at the bottom of the assignment page. It is an online submission with text entry. Here's a screenshot of the bottom of the assignment page.
@erica_wagner Your screenshot indicates that you're working in Edit mode. The +Rubric button only displays upon saving the assignment; it's not available in Edit mode. Try saving the assignment and see if the button displays then.