i cant download software update

Is your device compatible to update to the new version? See the article(s) below. 

IOS 15.1.1 Compatability

IOS 15 and iPadOS 15 Compatible Devices.  *

iOS 14 and iPadOS 14 Compatible Devices.  *

iOS 13 and iPadOS are compatible with these devices - Apple Support.  *

See the articles below.

Update your iOS device.  -

Get help with over-the-air iOS updates  -

Update – Can’t or can’t restore   -

Have you tried using iTunes on a computer or Finder if running Catalina or later to do the update?

Update software on iOS devices in iTunes on Mac. -

Phone, iPad, or iPod touch - Update with Finder.  *

iPhone, iPad, or iPod touch - Update with iTunes.  *

If you see an error when you update or restore your iPhone, iPad, or iPod    *

Updating iOS: common issues and solutions.  -

https://www.youtube.com/watch?v=csJAEyvvruY