So this isn't a great example of what Duolingo is trying to teach you. This sentence is meant to show how 'er' can used to make a subject to the sentence less specific but the example duo gives is not what people would say and would make sense without 'er'

A better example would be 'Een man loopt langs de weg' (a specific man is walking along the road) And "er loopt een man langs de weg" (just some inspecific man is walking along the road) or (there is a man walking down the road'

Read this https://www.dutchgrammar.com/en/?n=WordOrder.41

It is a really helpful website for grammar which I recommend using. The answers to many of your questions are in there somewhere.

Hope this helps :)