"Although he is sick, he did not take a day off."
It would make more sense if the sentence was in 雖然...但是.... form. So "雖然他生病了，但是他沒有請假。”
Always pair duolingo with some supplementary, explanatory material! You are not going to learn chinese, or any language, with duo alone.
In this case, the subject can go before or after 虽然. As far as 了，it can go at the end of either phrase, depending on the meaning. If both phrases are in the past tense, you only need to put 了at the end of the 但是 phrase.
If you're combining the tiles of words, then a second 他 is missing for this word order — 虽然他…但是他…
虽然他生病, 但是没有请假了 is WRONG. The suggestion is 他虽然生病, 但是没有请假了。
However when I follow the suggestion, it is still WRONG & the suggestion is 他虽然生病了, 但是没有请假。What the hell? DL is NOT CONSISTENT. Which is true???