The Stop task button is only visible after starting the task. This prevents testers from giving up without attempting to complete the task.
Stop a task on desktop
When testing on desktop, the Stop task button is displayed after starting the task — i.e. after clicking the Get started button to start the task, then clicking the prototype screen at least once.
Stop a task on mobile devices
When testing on a mobile device, the option to stop the task isn't immediately visible. Instead, a purple bar is shown at the bottom. If testers get stuck after starting the task, clicking the purple bar will reveal the instructions again, as well as the Stop task button.