"I do not go to the market on Sundays."
Translation:Saya tidak ke pasar pada hari Minggu.
'pada hari minggu' is better (it's the standard).
'di hari minggu' is not perse wrong, but it's not the standard, it's used in colloquial speech.
This course mixes up formal & informal, so I can imagine it's a bit unfair to reject the informal style when it's already used in other sentences as well.
I think the most accurate translation would be something like this :
Saya tidak pergi ke pasar pada hari Minggu.
I wouldn't leave out 'pergi'.
I also wouldn't leave out 'pada'.
But that's just my personal preference.