I can’t upgrade to iOS 16 ,why please or can you help to upgrade my iPhone 12 to to have iOS 16 and above .Thank you

You can update an iPhone 12 to iOS 17.6.1.

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

• You need at least 10 GB of free storage for iOS 16 and later or 6 GB for iOS 15 or lower; more is better
• You must be connected to a Wi-Fi network (and not a personal hotspot) unless it is an iPhone that supports cellular 5G (12, 13, 14, 15 or SE 3rd edition) and has a 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, and it may be necessary to delete any VPN apps and VPN profiles
• The phone cannot be in Airplane mode and must have a cellular data connection
• You cannot still have a beta of a previous version installed on the phone
• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it

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

Here are 2 links, choose the one for your computer.

• if you're using a Mac with macOS Catalina or later
•  if you're using a Mac with macOS Mojave or earlier or a Windows PC

remilekun243 wrote:

I would like to download some app on my phone but it keep say it use only iOS 16 and while trying to update I could not update it

You need free storage space

If you can't update or restore your iPhone or iPod touch Recovery mode

If you can't update or restore your iPhone or iPod touch - Apple Support

If your iPhone won't update - Trouble shooting

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

if you have an older version of iOS that was not installed on your phone—

Home Screen > Settings > General > iPhone Storage

Scroll to the bottom of the list to see if there are any old iOS update listings(?)

If yes then tap on the old iOS update, then tap to Delete

The iPhone 12 should update to iOS 17.6.1, the current version, and nothing older.

Which version of iOS does it have now?

Is it some app that says "it use only iOS 16", and when/where does it say it? Is it in the Apple App Store?