"I work a lot during the week."
Translation:Lavoro tanto in settimana.
The way to say this does not copy the structure of the English, it is one of those cases that come up so often when you learn a language: don't try to understand, just remember! Or switch off your "English brain" as thmarchi put it! If there is a reason, it must be something with "in settimana" meaning "every week or so", which makes it sort of indefinite.
I waited until I had contacted my teacher to answer this for you. If you have been following the other responses, you will see that there is often more than one way to express a situation in Italian, and so the more common way to express this sentence is Lavoro tanto durante la settimana. However the one that Duo uses, Lavoro tanto in settimana, is also correct, but would be more common in business or more formal speech. I have noticed often that this expression, in settimana, or" in città" etc, is often used, where in which a word or expression is implied but not used, as in this case. As Babiole says above, often we need to switch off our English brains and try to think like an Italian. I believe it is a simple way of saying in the week, and in this case the pronoun is not necessary, sort of like a shortcut. So if you were to try to translate the sentence literally it would be, I work a lot in the week, which is another way of saying , in the course of or during. I am glad though to see that duo is accepting the more common form of the translation. I am not a native speaker, I am American born, but of Italian decent, so I hope this helps a bit.
"...durante la settimana" means "during the week." I am flummoxed DL will provide the accurate translation in the drop-down menus, then not include that choice in the individual word choices below. It's no wonder I read about so much confusion in the comments. Taking a "close enough: approach in the initial stages of learning is more appropriate to throwing horseshoes than learning a precise skill from which one can eventually learn different approaches to the same end.