I can’t download the update iPad 16.5.1 (c) why not? ???

Not sure.

What is the rated, internal data storage space/capacity of your iPad model and how much of this free, internal data storage capacity is left inside of your iPad model?

All of that additional info can, also, be found in the Settings app, General settings panel, About.

Your iPad needs sufficient free internal data storage space. At the very least, 7-8 GBs or greater internal, free data storage space.

Are you using a dedicated Home wireless, WiFi network with a WiFi MODEM/router box serving as the connection to an ISP ( Internet Service Provider ) AND NOT a WiFi hotspot from a smart phone?

Any iOS/iPadOS upgrades/updates CANNOT be done over a cellular phone/data service using a smart phone WiFi hotspot.

You may need to find an outside location, that has free WiFi, that uses a dedicted wireless, WiFi MODEM/router box network, to the Internet, to do the iPad update.

FWIW, this update was only 8MB on my phone and on my iPad. Storage space should not be an issue. I keep reading other posts where users are having problems downloading the update. I’m sorry but I have no explanation for it.

Exact same problem for me, except on iPhone 14 Pro. iOS 16.5.1 installed, >300GB free space.

"Download and Install" is grayed out on the Software Update page and the message "This update requires a Wi-Fi network connection to download." is shown, even though I have a very strong WiFi connection.

iOS updates set to automatically download and install, Security Responses & System Files automatic updates are enabled. Have tried turning phone off then on.

I had no issues with either my 2015/2016 iPad Pro or my iPhone SE 2nd gen.

So, I am really not sure what issues are happening with this rapid security update, either.

I had no problems either.