How to track stolen iPhone

The only way the phone could be removed from iCloud is if you fell for this common phishing scam: You get a text or email from “Apple” or “Apple Security” (or sometimes “Police”) saying your phone has been found and to click on a link to see its location. This is NEVER from Apple or from law enforcement; it is from the thief or fence to steal your Apple ID and identity. Delete any such message without responding to it. Apple never gets involved in lost or stolen devices→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams

If this is what happened then in addition to not being able to track your phone, all content in your iCloud sync or iCloud backup is now in the hands of the criminal who stole your phone, and Activation Lock has been removed, so your phone can now be safely resold.