I think that might be the warning that pops up if you try and attempt a lesson halfway through a section. In many cases you have to complete lessons in a linear fashion within each section (that might not be the correct terminology but it’s something like that). So far as I’m aware Get Started should lead you to the start of that particular section which you could then work through until you reach that particular lesson.
You could check the syllabus page which may have details on where this lesson is located within that section or try
get started and click the 3 straight line menu icon in the bottom left hand corner which shows slightly more localised details/order of lessons. Chances are you’ll still need to complete a few lessons before reaching either of those two.