what requirements I need to bring my 15 years old niece to France
 

I want to bring my 15 years old niece to France, I read they need both parents consent, but one parent we don't know where it is, can we have just 1 parent consenting or what other requirements do I need to enter Paris France Thanks for answering

Surely the available parent has legal custody of the child. That should be sufficient. The problem may be more leaving the States than entering France.

A notarized letter from the parent who has legal custody. Medical insurance information wouldn't be a bad idea, either.

Google " single parent travel consent letter" and there are forms that you can
print & fill out for traveling with a minor. A notary will be needed. I did this for
my up coming trip with my 15 year old niece.

You need to be sure that the available parent has sole custody. If not - you will have to have permission from both parents - as well as approval to oversee the child's medical care - and naturally her passport. Also assume they will want to see her return ticket to know she will be returning to the US within the 90 day tourist visa limit.