Click on the link for an explanation of that message in the article 'About genuine iPhone batteries' --> About genuine iPhone batteries - Apple Support
"An Unknown Part message will appear if the battery installation is incomplete or if the battery:
• Was replaced with a nongenuine battery
• Was already used or installed in another iPhone
• Isn't functioning as expected"
Get Apple to examine your phone.
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
For the iPhone Service Answer Center click here --> Choose Your Country or Region - Apple Support