I lost my iphone.

To locate it in the Find My [Device] app, you must have activated the Find My app BEFORE the iPhone X was lost. Here is some information on that app ➔ If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support.

Note that your lost iPhone must be able to make the connection to the internet via Wi-Fi or cellular connection so that it can mark its location and broadcast it. 

Axel F.