Is this the equivalent or similar to the English phrase "just a minute"? Used when someone asks you to do something but you are currently busy?
That would be probably "za moment" "sekundę" "minutkę"
"to tylko sekunda" could work in telling you "it will take only a second", but "minutka=little minute" or "chwila/chwilka" (a moment) would work better.
In conventional English, there is no ’It is‘ preceding ‘Just a second’ so if time is implied, rather than rank (2nd of 8, etc.), different hints would be better.
But it's not "Just a second" as in "Wait a second" (is that what you mean?)
It's a simple declarative sentence which means exactly "This/That/It is just/only a second".
It would be more common in English to say, "It takes only a second," than the translation in this exercise
That actually makes sense to me, looks like a probable interpretation. Added.
Does Duo automatically understand shifted word order? Because "it only takes a second" is probably a more common word order
Very few things are understood automatically by the algorithm and shifted word order is definitely not among them :)
Added "It only takes a second".