i cant download iOS15

iOS 12.5.6 is as far as the iPhone 6 Plus can go. It cannot be updated any further.

If you need to run iOS 15.7, you will need a newer phone

If you need to run iOS 16, you will need an iPhone 8 or higher.

An iPhone 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.