One of the key accessibility features for any application is keyboard navigability. Because many forms of assistive technology rely on keyboard controls exclusively, testing for keyboard support is a good thing to prioritize. Check in particular:
Can basic tasks and workflows be completed using only a keyboard?
Can all forms be completed using only a keyboard?
When an element has tab focus, is there a visual indicator?
If a modal or overlay opens, is keyboard focus trapped inside the new context until it is closed?