The city police have decided.
what is the error in it?

I think your question is incomplete. And if it is complete sentence then no error is here.

i think instead of have ..has will come

has will come.
 

'THE CITY POLICE HAS DECIDED.' will come instead of your question.

↵According to me, there is nothing wrong in this sentence. I had discussed this issue with a Prof. of English literature at Oxford University. He said that "In short, ‘have' is correct and ‘has' is wrong in most British English usage."

Please search for "Matching verbs to collective nouns" and you will be directed to the website of oxford dictionary.