I can’t see my battery health

Battery health uses the battery's built in diagnostic. If it isn't reading (as yours isn't) that means either the battery has failed or an original Apple battery was replaced with a cheap knockoff. But it will only cost you $49 to replace the battery (assuming there is nothing else wrong with the phone).

Your iPhone 6 battery needs a checkup.

Make a genius bar appointment with Apple for an evaluation.

They have diagnostic tools and can tell you what options you have.

Settings > Battery > Battery Health > here.

If you have an iPhone 6 Plus, maybe you need to replace the battery?

Even if you tap on the ">" symbol on the right side?

Regards

Giulio