My iPhone 6s Plus iOS 13.5.1 is not installing iOS 15.4

What do you see on the screen? What makes you believe it is not installing? See this, If your iPhone, iPad, or iPod touch won't update - Apple Support or this, If you can't update or restore your iPhone or iPod touch - Apple Support

1. You must have at least 6GB of free space (Settings > General > iPhone Storage)
2. You must have a good WiFi connection.
3. You must have a 50% battery charge and preferably connected to power.
4. Low Power mode must be turned off.
5. You cannot be using a VPN.
6. If you have a beta profile on your iPhone you must remove it.

For downloading or for installing cause I had over 6Gb before downloading and I’ve also free up some space to 3gb for installing