Can I track my iPhone 15 if it is powered off?

When you power off the iPhone, you see the message that the iPhone is trackable even when turned off. If it has been longer than 24 hours, you should still see the last known location and the location will update the next time it is able to connect to the internet. You will have to keep checking. A determined thief will remove the SIM card or put it in a faraday box to prevent any outside connection, but they still will not be able to access the device that is locked by the passcode. Watch out for any fake messages that you may receive that claims to be from Apple asking for your Passcode/Password.

Then there's nothing further you can do to locate it unless the iPhone reappears in the Find My system. Report it and set it to erase per the instructions in the support article Kurtosis linked to above. Change passwords to your Apple ID and any financial accounts or other critical information you may have had via apps or links on the iPhone, just as a precaution.

Regards.

That is correct, if the phone is off, you won't be able to track it.

What to do if your phone is lost or stolen.

If your iPhone or iPad is lost or stolen - Apple Support