For the record, it’s not quite an error as a difference in culture.
Think about these two phrases in English.
I think he isn’t happy.
I don’t think he’s happy.
Which sounds more natural?
Most likely the second does. Just as in Japanese they would say
“彼は嬉しくないと思う。” instead of “彼は嬉しいと思わない。”
It’s just a matter of where they put the negative. In this case it doesn’t change the meaning at all.
A decent understanding of English grammar is required if you continually compare English grammar and Japanese grammar. Otherwise there’s no benefit from comparing them. You’d be better off just learning Japanese grammar in Japanese.