Welcome to the Community @Virdjinija! The Fitbit app is mainly designed for the iPhone, that's the reason why it does not appear. What you need to do is go to the search option and type on the search bar: "Fitbit" and then in the left upper corner it select the option "iPad only" and switch it to "iPhone Only". The Fitbit app will appear and you will be able to download it.
I hope this helps. Let me know how it goes.
