Hello valbona132,
Welcome to Apple Support Communities.
It sounds as when loading up your new iPhone, you are missing the calculator and unable to re-download it from the App Store, correct? We all use the Calculator app all the time, understand your concern. We're here to help you take a look in to that!
First, we would recommend is checking the Screen Time settings for restrictions. Head to, Settings > General > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > Allow
Here are some article to walk through on step to resolve this issue:
Use Screen Time on your iPhone, iPad, or iPod touch
Download apps and games from the App Store
Hopefully this helps!