Just get use to Korean grammar rules.
Subject -object - verbs
1- The People - persons - are.
(The people are person )
2- in Park - nobody - there is not.
(There is nobody in the park )
Stop thinking in English grammar rules.
Why does it no accept '사람은 공원에 없습니다'? In the previous examples both '는/은' and '이/가' particles were accepted (ex. 방은 집에 있습니다 and 방이 집에 있습니다 were both correct) since without a context it's hard to tell whenever topic or subject particle should be used...