I can’t download App on mr App store

1. This is a user to user forum. As users we cannot update anything for you. Go to Settings->General->Software Update to update your device.
2. What model of iPhone do you have? If it's running iOS 12.5.6 it is an iPhone 5s, 6 or 6 Plus and cannot be updated past iOS 12.5.6. It lacks the hardware to run anything newer than 12.5.6.
3. If you need an app that requires a newer iOS version, then you will need a newer iPhone model.

If you have an iPhone older than iPhone 5s, iPhone 6, iPhone 6 Plus you cannot update to iOS 12.5.6.

You can update your iPhone to iOS 12.5.6, the last security updates iOS 12.5.6 -> released August 31, 2022.

Available for: iPhone 5s, iPhone 6, iPhone 6 Plus

Go to Settings -> General -> Software Update -> Download and Install.

Yes, most apps require iOS 13 or later.                                

Considering a newer type of 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