Translation:My friends were traveling, and I was working.
Does "a" in Ukrainian function the way it does in Russian? That is to say, introducing a contrast between the two things it connects? I translated this as "My friends traveled, but I worked."
"My friends traveled, but I worked." скоріше перекладеться як "Мої друзі подорожували, проте (але) я працював"
But in a similar sentence (You like eggs, but they like mayonnaise) you get marked wrong by duolingo when you use "але", they require "a" exactly because of that contrast construct. So I find it unfair that in the sentence here, "but" is not accepted.