Hello oras78,
We suggest making sure that you're running the latest iOS version (currently iOS 16.0.2). You can do so by tapping Settings > General > Software Update. Software updates are designed to fix bugs and to enhance the user experience, so it may help to resolve the issue you're experiencing.
Update your iPhone or iPad - Apple Support
Once updated, test to see if you get the alert to enable Low Power Mode:
Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support
If you still don't get the alert, contact Apple Support. They will have additional resources to assist further.
Take care.