There's two reasons why you might not be able to able to access Quiz mode for a section.
1.) The section doesn't have enough valid questions for Quiz mode. If the section has a fewer than three interactive questions quiz mode will not appear for that section.
2.) You haven't studied that section yet. You can only access quiz mode if you've already studied the section once in adaptive mode.
What to do when Quiz mode isn't available?
When Quiz mode isn't available it is best to study the section in adaptive or one of the other premium modes.
Quiz mode is a premium feature. Anyone on the Exam Boost package or above can access quiz mode.