Though the sentence is not correct. You can write it as "how come a beautiful girl like her is single".

mengapa seperti seorang gadis cantik tunggal. A man said that to you on a chat site? he is trying to gombal you.

This is not a complete sentence. Maybe you took it from a longer sentence. 'I can't understand how such a beautiful girl is single' makes sense and means why is this beautiful/pretty girl not married or why doesn't she have a boyfriend?

how come a beautiful girl like you is single? *if this was a message to you