My iPhone doesn’t update to iOS 15

that model is not an iPhone 11. it should be an XR.

Identify your iPhone model - Apple Support

iOS 12.0 is what it shipped with

https://en.wikipedia.org/wiki/IPhone_XR

The phone has literally never been updated?

Probably use a computer to update it. Here is a good user tip of other things to try

Updating iOS: common issues and solutions - Apple Community

go to Settings > General > About

What is your iPhone Model and Software Version?

If your phone says it's up to date at iOS 12.5.5, then you have an iPhone 6 model (or older) and you cannot update beyond iOS 12. Time for a new iPhone model if you need to run newer iOS.

abd4137 Said:

"My iPhone doesn’t update to iOS 15: When I go to settings and software update my iPhone says I’m updated to Ios12 and there is no update to do."

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I see you have an iPhone 11. So, it is compatible.

To Check for Updates:

Use a Computer and Ethernet:

Start with attempting an update through use of a computer. If you cannot use Wi-Fi, then use Ethernet.  Go Here: Update your iPhone Using a Computer - User Tip. On the computer, you would connect it through Ethernet, going directly into your router. For a Mac (or PC) that does not have Ethernet, this would be the adapter to use: Belkin USB-C to Gigabit Ethernet Adapter - Apple. If you get the same error, the model is likely just not compatible.

The iPhone 11 came with iOS 13 so you definitely don't 'have an 11.

You said it's an A1984, that is an iPhone XR, not an 11. the XR came with iOS 12.

You seem to be confused as to what model you have. Follow Chris's instructions on how to find your model number. I suspect Chris is right about it being an iPhone 6.

I don't think your screenshots are adding any information here..

As I suggested, use a computer to try and update.