Why can’t I turn off “Find my iPhone”?

No, Find My iPhone is on all the time if you turn it on, nothing has changed. You seem to have bigger problems than just this, did you try what I suggested and also turn off low power mode.

If you’ve gone through the above suggestions and all that also including switching to and using high-powered mode, best contact Apple Support.

I did so on my iPhone 13 Pro Max yesterday.