What can I do when "Find My iPhone" won't turn off?
Enter the Settings menu of your device.
Tap the name that you see at the top of the menu.
You should see your available devices and the ability to change personal settings. Tap iCloud.
Scroll down until you see “Find My iPhone” or the green radar icon and tap it.
You can toggle the feature on and off here using the button labeled “Find My iPhone”.

https://www.ubreakifix.com/blog/turn-off-find-iphone