15.4 won't install on Iphone 12, ipad, or ipod

To update over the air from the phone all of these requirements must be met:

• You need at least 6 GB of free storage
• You must be connected to a Wi-Fi network (and not a personal hotspot) unless it is an iPhone 12 or 13 series with an active 5G network connection
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN
• The phone cannot be in Airplane mode
• You cannot still have a beta version installed on the phone

If you can't meet all of them you can still update using your computer.

The above is the short answer. Here is the long answer→Updating iOS: common issues and solutions - Apple Community

Hey there,

See ☞ If your iPhone, iPad, or iPod touch won't update - Apple Support

If your iPhone, iPad, or iPod touch won't update - Apple Support

Are the devices under Mobile Device Management (MDM), under a business or school plan?

Have you tried installing it with iTunes on your Windows 10 computer (do not use safe mode)?

wd513 Said:

"15.4 won't install on Iphone 12, ipad, or iPod: IOS 15.4 extracts software, prepares update then fails to install on all three devices for 'unknown error.' I have itunes and windows current. My wifi and ethernet connection are fine. Why would it fail on all of them, and how can i install the upstes?"

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Use Safe Mode:

There is apparently an error connecting to Apple's servers. So, boot into Safe Mode on your computer, and then perform the update using a computer.

Try my User Tip: Update your iPhone Using a Computer - Apple Support

Booting in Safe Mode: How to Use Safe Mode on your Mac - Apple Support

With all due respect, Windows does have a safe mode, however I have no idea why it was mentioned as it's not going to connect you to Apple servers any differently than normal mode and may actually hinder the process as safe mode loads a minimal set of drivers and may not communicate properly with the phone.

How to boot in Safe Mode in Windows 10

1. Hold down the Shift button as you click "Restart." ...
2. Choose "Troubleshoot" on the Choose an option screen. ...
3. Choose "Startup Settings" and then click Restart to get to the final selection menu for Safe Mode. ...
4. Enable Safe Mode with or without internet access.

Lawrence Finch Said:

"Windows doesn’t have Safe Mode."

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"itunes and windows current".

If a currently-opened window of iTunes on a Mac, then it would apply (that's how I've interpreted those words).

wd513 wrote:

I have itunes and windows current.

Does this mean you’re using iTunes 12.12.3, and Windows 11?

Did you verify you comply with all of the requirements for update that I posted?

Windows doesn’t have Safe Mode.