I cannot access my battery health in my iPhone

There are two reasons why the "Unknown Parts" message might appear.

1) You have had the battery replaced and an unauthorized shop installed the wrong battery.

2) You still have the original battery, but the phone has detected a possible problem with the battery.

Does your phone still have the original battery?

Either the battery was replaced with a non-Apple battery or the battery is failing. You should know which it is.