Can I track my iPhone if it is turned off?

No.

If you are seeing an "Offline" message when you are using Find My to try to locate the phone, this indicates that one or more of the following are true:

The battery has run down

The phone has been powered off

The phone does not have an Internet connection where it is located

If you feel that the phone was stolen, iPhone thieves are very fond of placing a stolen phone in a special type of small bag that will not allow the phone to ether send or receive a signal.....even though the phone might be working just fine otherwise.

Unfortunately, there is nothing that you can do about this other than keep checking a few times each day to see if the status of the phone has changed.

If the status of the phone has not changed in a few days, you have to be realistic about things. The chances that the phone could ever be recovered are just about zero at this point.

There's a slim chance – emphasis on slim.

The Internet is by far the most likely way for your phone to be able to report its location. But for that, it needs to be turned on, and connected to a Wi-Fi hot spot, cellular data, or both.

The not-so-dependable backup?

If you had Find My Network turned on, and the phone has some battery charge left, it will send periodic Bluetooth beacons even when it is powered off. If another Apple device comes close enough to the lost/stolen phone that it can hear them, it may relay the location of the phone using its Internet connection.

This won't help if your phone is completely out of power, or if thieves have placed it in a Faraday cage type of bag that blocks radio signals (including Bluetooth signals) from getting in or out. And the chances of someone coming close to a stolen phone are lower than the chances of people with iPhones coming close to a lost AirTagged bag of luggage in a busy airport.

Bob Timmons wrote:

If the status of the phone has not changed in a few days, you have to be realistic about things. The chances that the phone could ever be recovered are just about zero at this point.

Even if you see the location of the phone, it would only. make sense to go there if you feel that the phone was lost, not stolen. You lost your phone at the mall, you see it showing up on the map at about the location of such-and-such department store … sure, it makes a lot of sense to go there, and to check with the store clerks or to search where you think you left it.

Going to a place where criminals gather to try to get your phone back could result in losing something much more valuable than your phone – your life.