I changed the battery for a new one but can’t still see my battery health why

Did Apple change the battery or did you use a third party repair service? If the new battery is not an Apple approved replacement then battery health won’t work. You could even get notifications that the battery cannot be verified.

If Apple did replace the battery then take your iPhone back for service.

This sounds as if you did not have an Apple Store or authorized Apple Servicer replace your battery. Is that a possibility?

If yes, a genuine Apple battery was not installed in your phone, so Battery Health will not work.