"I go shopping on the weekends."
To my knowledge かいものに行きます would be absolutely fine, Duo's just trying to teach you another way of approaching this sentence.
Here it's using します which essentially means, 'do'
So this sentence reads more like 'i do shopping on the weekends'
If we use the WA particle on weekend, wouldn't that mean its the weekend that goes shopping rather than I?
In this case, "は" is translation for "on". "I"(私は) is omitted in this sentence. If I write this without omission; 私は週末は買い物をします。 or 私は週末に買い物をします。
I literally put in what it said was the correct answer and I got marked wrong. Reported it.
It's telling me the kanji I chose is wrong. It's the same kanji as the correct answer, and now I cannot complete this unity because of that