I’m trying to download iOS 15 on iPhone 6

iOS 12.5.7 was the last update for the iPhone 6 Plus.

No more updates are available for the iPhone 6 Plus.

If you want to run iOS 15, you need an iPhone 7. If you want to run iOS 16, you need an iPhone 8 or higher model.

Sambasha100 wrote:

Try to download iOS 14 on iPhone 6 Plus

You can't.

For the iPhone 6 Plus, iOS 12 is the end of the line. The latest release of iOS 12 is iOS 12.5.7.

About iOS 12 Updates - Apple Support