It may take some time to train your brain to hear it, but it is an "L" sound.
I remember when I first started speaking Spanish with native speakers I had trouble distinguishing the words "lo" and "no" (which have very different meanings). But after a few months, I wondered how I ever got them confused. They no longer sound at all alike to me.
In Spanish the L sound is harder than the English one, and that may be why I confused it with N. It may be something similar for you in Greek. English teaches us that if it's not a liquid sound, it can't be an L, so your brain tries to come up with a different sound.
But don't worry, with practice you'll start catching on to the pronunciation.
Try saying "lu" but make your tongue tense. Practicing that will help.
I was so keen to learn Greek, but I am insanely bored with the content and the monotone. The other Duolingo courses are wonderful, but this one is so repetitive. Am I the only person having difficulty persevering with the Greek course? Is there anything to supplement it and liven it up?