Why didn’t see the battery health?

Assuming you have not had the battery in your iPhone replaced by anyone other than Apple or an actual Apple Authorized Service Provider, you open the Settings app and scroll down to Battery > Battery Health and Charging to see the health of your battery.

If you had the battery changed somewhere else, you won't be able to check battery health, because you wouldn't have an authentic battery in your iPhone.

What do you see when you attempt to check Battery Health? What version of iPhone and iOS do you have? Settings>Battery>Battery Health & Charging>Maximum Capacity.