Is there a way to completely remove and disable find my iPhone?

you cannot delete Find My, but you could remove it from your Home Screen

Remove apps from iPhone - Apple Support

you could also go to Settings > [your name] > Find My > Find My iPhone and turn it off

Turn off Find My on your iPhone, iPad, or iPod touch - Apple Support

Please explain what’s wrong.

Only the owner of the account used to sign into iCloud and enable find my can turn it off.