My iPhone 15 says unable to determine if battery is genuine

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

From your not charging report I'd say the third item is a possibility. 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

Hello~ Where did you buy this iPhone and when? From Apple? Have you had the battery changed by someone other than Apple ?

~Katana-San~