It should not be accepted indeed, because it won't sound the same (no liaison between Jean and achèterait and you really need en in that sentence. Even Jean en achèterait would sound different. But that's funny.
I dropped the preposition in front of Sunday: "Sunday I'll buy..." Duo insists on "On Sunday..." Both "Sunday" and "On Sunday..." are correct in English to indicate "this coming Sunday" rather than "Sundays". Boy oh boy, this is ever so frustrating.