Can’t download apps on iPhone 6

You have a very old iPhone, which is limited to iOS 12.5.7. If you are trying to download any apps which need a newer version of iOS, you'll need to buy a newer, more modern iPhone.

When you try to download an app, what is the error message you see on your screen?

Do the apps require a newer iOS version than your iPhone 6 can provide? Probably yes.

Frankly, you need a newer phone. An iPhone 8 can run iOS 16.