Why can’t I download the new update

Try to update using another Wi-Fi. If still no luck, you can try to update using a computer: https://support.apple.com/en-ie/HT204204

There might be some temporary server outage too as you are not the first person today with the same issue.

First let's make sure you have a 6s Plus.

Settings>General>About

What's listed for Model?

~Lyssa