16.4.1(a) Unable to Verify Security Response

@sberman has researched the issue. Apparently it is because the update is intended to be installed automatically, with a staggered rollout. So if you manually try to install it you may get that message. This is from AppleInsider:

While the update is seemingly available to download straight away, you won't necessarily be able to install it. [Analysis] reveal that there is a "granular ramping logic," in that it will allow a percentage of users to install

the update at a time.

———snip-------

Only 5% of users will be able to install the update in the first 6 hours of release, rising to 15% by 12 hours, 40% by 24 hours after release, and 70% by hour 36. After 48 hours, all users who downloaded the update will be able

to install it.

———snip-----

Attempts to install the update before the device is allowed to will display a notice saying the iPhone is “Unable to Verify Security Response" and claiming it is because the device is "no longer connected to the internet." Despite the apparent lack of internet according to the message, users can still go online normally with their iPhone without restriction, and the notice only applies to the update.