How do I recover my unavailable iPhone 6 my niece is locked me out of it and I had already deleted it off my find my iPhone app

Lock Screen Passcode or Screen Time Passcode.

Following this process is the only way out if your iPhone is disabled. The steps given in this link will help you reset the passcode -> If you forgot the passcode for your iPhone, iPad, or iPod touch, or your ...

Note: If you have forgotten the Screen Time Passcode then restoring your device from a backup after the passcode reset will return the Passcode back to your device. So, please do not restore from a backup.