What is wrong with "We are down in the cafe" down and below should be interchangeable
In the UK a cafeteria tends to be the eating place within a place of work. Cafe is the usual word used when in town.
Good question! It looks as though abajo means "down," "downstairs" or "below," and debajo [de] means "underneath." So, in Duo's sentence ("We are downstairs, . . . "), it seems as though abajo is the correct choice, But, if it were "The red book is underneath the blue book," debajo de (El libro rojo está debajo del libro azul) would be correct.
I know this doesn't have to do anything with the sentence, but this is probably the first Duolingo lesson node that has a large amount of new vocabulary in it, and not just obvious new conjugations for words that were learned a while back.