in Turkish siz vs sen
both of these mean "you" but when do you use "siz" when do you use "sen"??
Depends on the suffixes you use.
'Sen okula gidiyor musunuz?' is incorrect.
Because; you used 'sen' and you must not add the formal suffix '-uz'.Just without suffix 'musun?'
'Sen okula gidiyor musun?' (Do you go to school?)
'Siz benimle gelir misin?' is also incorrect because you used 'Siz',the formal way of saying 'you' and you have to add '-iz' at the end here.
'Siz,benimle gelir misiniz?' (Can you come with me?)
Conclusion; Use suffix in formal speech,and dont use in informal one.
Thus,i suppose that's why duolingo marked your question as wrong,you might have used wrong suffix.
If the problem is not the same as i explained,type the problem,especially the Duo's sentence so i can help you
As has been said, "sen" is the informal singular "you." It's for people you know well, small children, etc.
"Siz" is the formal singular. BUT it's also used any time "you" is plural. You could be talking to little kids, your siblings, whoever: Even if you'd normally address them informally, if "you" refers to more than one person, you use "siz." There isn't an equivalent of the German "ihr" for informal plural "you."