I can’t update my iPhone 6s Plus to iOS 15.4!

ldean9574 wrote:

It’s currently 12.4.1

Because you are so far behind with the version of iOS, you may need to use iTunes on a computer to complete the update.

Update your iPhone, iPad, or iPod touch with iTunes - Apple Support (CA)

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support (CA)

If it's currently running iOS 12.5.5, it is NOT a 6s Plus. It is a 6 Plus.

The user tip below may offer clues.

Updating iOS: common issues and solutions - Apple Community

What happens when you try?

Any error message?

What is the existing version of iOS?