@44m1r, no. In Spanish "el" goes before a week day, however "en" is implied (which would help with the literal translation in english) but you are learning Spanish, not english so the translations/expressions won't always be like for like, nor will they necessarily make sense if translated as such. Hope that helps.
There is going to be a problem when the English can mean either of 2 meanings. Do you work on Sunday can mean Are you working on Sunday, but usually it means do you work on any Sunday. The better Spanish would be los domingos, but I can see why they keep it easy for us, kind of!
Do you work on Sunday(s)? is used in British English to mean usually, regularly, normally; in other words, it is used for recurring actions. If you say "Are you working" it means at this moment, currently. "Are you working now?" or Are you working today? (if you meet someone) are the usual ways to express "currently occupied, busy at this moment". English also uses the present continuous to encompass the past/present divide. So "Are you working at the moment?" can mean "Have you got a job now? Are you in employment?". The present continuous can also refer to the future "Are you coming tomorrow?"
Although the English simple present is not so forgiving. It cannot express a continuous action in the present, only a repetitive one. Without an indication of repetition in an adverbial of time (on Sundays, usually, every weekend, all the time, each day) Tu trabajos el domingo? can only be "Are you working on Sunday?"
The only way Do you work on Sunday? can be right is if you accept on Sunday as an adverbial of time equivalent to on Sundays, and many English speakers do not.
Then there was a bug in Duolingo or a spelling mistake in your answer. Duo doesn't pay attention to capitalization. I'm too lazy, so I type the answers nearly always without capital letters. Dou reminds me to pay attention to the spelling, but the answers are still counted right.
Earlier today i translated this by adding an "s" to sunday, to see if it would be accepted. "Do you work on sundays?" And i got it correct. I just did the same and got it wrong......... Normally when i test out of levels i dont get any wrong. Now i have one wrong and im not getting my gem thingy at the end........