Some applications, such as Perusall are automatically hidden from students. To make it visible and enable it for use,  move Perusall to the main navigation area. 

Step 1: Open your Canvas course

Step 2: Click settings at the bottom of the navigation.

Step 3: Open Navigation 

Step 4: Use your mouse to drag Perusall to the main navigation menu. 

Step 5: Click Save

Step 6: Perusall is ready for use is is listed in the main navigation of your course.