How can I trace my iPhone when it is off?

Simple answer is you cannot trace it. It must be powered on, and it must be connected to the internet. When powered off, an iPhone is just an inert hunk of glass, plastic and metal. It’s as remotely trackable as a stone would be.

XFactor0269 wrote:

I read that those can be traced even when off,

iPhone does not offer that capability, irrespective of whatever you're reading.

It is common for stolen devices to never be brought back online.

As it gets asked, no, there is no way to enable Find My remotely, either.

And as it gets asked, hopefully, your device passcode was robust, as that's what is now protecting your data here.

What to do? You have been provided with a link to what you will want to do here.

Click or tap on the following blue-text link for what to do now > https://support.apple.com/en-us/HT201472

Apple does not get involved with lost or stolen iPhones, nor with found or "found" iPhones.

XFactor0269 wrote:

How can I trace my phone when it is off?

Not powered off.

There is a feature for when a device may be "offline" but not off.

Perhaps that's the confusion.

It is also quite likely by now the thief has already erased it by restoring it. That breaks the Find My remote tracking connection so it can no longer be tracked remotely.

But, as long as it remains listed under your account as a trusted device, Activation Lock remains in effect and will prevent anyone from activating and using it.

This article should help:

https://support.apple.com/en-us/HT201472

As Michael has stated in the above post, you cannot trace the phone unless it is on and connected to the internet. It has to be turned on to send a signal indicating its location.