"I bought a lot of clothes at the department store."
When you put たくさん in front of the noun you should also put particle の, eg. たくさんのふく.
Or, more accurately, "By going to the department store, I bought a lot of clothes." It's easier to think of the logical order and also know about particle placement:
(noun)を(action on noun)
(place/noun)で(action performed on/at noun/place)
In the section instance, ふく and たくさんかい work as the action (buying lots of clothes)
Or, more accurately, "My clothes were put in the many department stores I bought."
This is the closest readable translation into English.
Unlike other languages, rearrangement isn't easy with the basic level of learning Duolingo gives us. Usually, を goes near the end of the sentence.
This is a very detailed explanation, thanks for the post. I'm not sure I follow how
translates to "my clothes were put in the many department stores I bought" since the を particle indicates that the clothes were bought regardless of if the order is incorrect.
Perhaps you mean the above translates more like "I bought clothes in many department stores"?
で is used to indicate the place /where the action is performed/. に is mostly used to indicate movement towards a place (plus other uses)
The model answer has unseen Kanji without explanation? Not so helpful. デパートで沢山ふくをかいました