Hello philippa10,
Restrictions are found in Screen Time settings on your iPhone. If you have this enabled with any restrictions for apps or content set, it could keep you from accessing the Health app. If you manage your own device, you can use the Apple Support article, Use Screen Time on your iPhone, iPad, or iPod touch, to check for restrictions.
Go to Settings > Screen Time to see if it is enabled.
If it is, look for settings in Allowed Apps and Content & Privacy Restrictions that may impact your ability to download apps or make changes to your device.
If your device is managed with parental controls, you may need to ask your parents to help.
All Our Best.