Why my iPhone can't update?

Hello. Your iPhone 6 Plus is only able to run iOS 12.5.5 at the latest. If you would like to run a newer version of iOS, such as iOS 15, you will need to upgrade to a newer device. iOS 15 is compatible with these iPhones.

An iPhone 6 Plus cannot update past iOS 12.5.5. You need a new phone.

You can't. Your phone does not have the hardware needed to run a newer iOS version. As others have mentioned you can't go higher than 12.5.5 on that old phone.