iPhone 14 keeps shutting off with a “Battery not recognized” message

That would indicate a hardware fault. Make an appointment at the Genius Bar of your local Apple Store or take it to your local authorized service provider.

Assuming you purchased it from Apple or an authorized retailer, it should be covered under warranty absent any physical or liquid damage.

You can check the warranty status here: Check Your Service and Support Coverage - Apple Support

If it's only 7 months since purchase AND it was purchased from Apple or an Apple Authorized Seller, then it is still within warranty and an appointment should be made at your Apple Store Genius Bar for evaluation.

Genius Bar Reservation and Apple Support Options - Apple

Who did you purchase the phone from exactly?

If you don't have AppleCare, and if you purchased it brand new 7 months ago, you are still in the warranty period. Book an appointment at an Apple Store or some kind of Authorized Service Provider.

Please note that a non-authorized (this includes both Apple stores and ASPs) service provider may install a battery that does not function well with the iPhone.

An Apple Store or ASP (Authorized Service Provider) by definition are authorized by Apple.

What you likely meant to say is that a store or service provider that simply claims to be an Apple "specialist" or "experienced with Apple repairs" is very likely not really authorized by Apple.

It's easy to find an Apple authorized shop. Just click on the link below to find authorized locations in your area.

Find Locations