Stolen unopened iPhone x

If that iphone wasn't activated or "find my iphone" was not enabled, there's no way to lock it remotely. Sorry.

Read this support guide:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Regards

Giulio

You would have had to open it, and sign in and turn on "Find my iPhone". If it wasn't opened and removed from the box, there is going to be no way of tracking it down and there will not be a way for you to deactivate it as it was never activated. Whoever got a hold of it, is going to be able to use it as a brand new phone like you would have when you opened it and started using it.