Why am I being told that my iPhone 14 pro max battery is not genuine

Read about that message in this support article --> About genuine iPhone batteries - Apple Support

A message about a non-genuine part is discussed in this support document: iPhone parts and service history - Apple Support

An Unknown Part message appears if the part installation is incomplete or if the part:

Was replaced with a nongenuine part

Was already used or installed in another iPhone

Isn't functioning as expected

Go to this page to find your nearest Apple Authorized Service Provider (AASP) or Apple Authorized Distributor (AAD) --> Find Locations

How to make a Genius Bar appointment

1. Go to Genius Bar Reservation and Apple Support Options - Apple

2. Go to "Get hardware help".

3. Select your hardware and continue selecting options requiring replacing parts until you see "Bring in for Repair"

4. Sign in with your Apple ID