If the "Start Quiz" button is missing (both in a normal browser and LockDown Browser) in Canvas,  it's due to a Canvas bug with the start date for the course.

The default start date for the course comes from the term settings, but the start/end dates set at the course level are supposed to override the term dates. Canvas is wrongly referencing the term date rather than the date set at the course level. This problem will typically only occur for courses where the instructor is trying to get students to complete a quiz before the official start of the semester or term. 

Example: The term is set to start on 5/10/2016. However, the instructor has set the start date for the course to 5/3/2016 to allow early access to the course.

To work around this bug, you may need to do one or more of the following:

* Set the start date for the term to the present date.

* Ensure there is a start and end date for the course.

*  Select the “Users can only participate in the course between these dates” checkbox.