The iPhone uses Bluetooth to send out a very brief intermittent signal after the phone has been powered off.
So, you would want to make sure that Bluetooth is set to On when you tap Settings > Bluetooth.
If the Bluetooth setting is "grayed out" so you cannot move the slider from Off to On to activate the setting.....that is a different issue.
Tap Settings
Tap [Your Name] near the top of the screen
Tap Find My
Tap Find My Phone On
Make sure that Find My iPhone is On
Make sure that Find My network is On
Make sure that Send Last Location is On
Report on your results.