Why does it say unknown part?

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 (e.g., you had it repaired with a third party service that did not use an Apple component)

Was already used or installed in another iPhone

Isn't functioning as expected

About genuine iPhone batteries About genuine iPhone batteries - Apple Support

Unknown part important battery message

Did you have the battery replaced recently? If yes, the shop installed the wrong battery. Post back if that might be the case for more information.

If the battery is still original, make an appointment at the Apple Store or take the phone to an authorized Apple Service shop so they can run some checks on the battery.

You can use this link to find an authorized service shop in your area: Find Locations