Translation:I have not been understanding the lessons.
It is impossible for the student to know which translation [have not understood... or have not been understanding ...] is required. Both make sense and both should be accepted in my view.
This Portuguese sentence actually sums up my recent experience quite well, given the way the tenses are translated into English!
Don't worry, folks, it'll get better. If you get through this set of lessons, the rest is a piece of cake. And it's better for your blood pressure, too!