Am unable to download the Facebook app on iOS 12.5.6. help me to download it please

Sorry, Facebook requires iOS 13.4 or later.

Yes, most apps require iOS 13 or later.                                

Considering a newer type of iPhone?

Before you buy a 'new' iPhone, read  -> The All Too Common SAD Reality of Buying a Used iPad/iPhone

The All Too Common SAD Reality of Buying … - Apple Community

Apple Store -> iPhone - Buying iPhone - Apple

Apple Certified Refurbished

https://www.apple.com/shop/refurbished/iphone/iphone-12-pro