This is (unfortunately) much more complicated & frustrating than suggested
This issue is endemic to Touch Bar MBPr (2016 & 2017)
Possible cause: Replacement LCD or Touch Bar (unsure if OEM or 3rd party matters)
Units with OEM replacement or never repaired are (likely) due to the T1 chip
Severity varies in my experiences:
a. 2017 15in #1 -- only installs the update after a series of bizarre steps (req on all reboots)
b. 2017 15in #2 -- automatically installs update on EVERY REBOOT.
Inference of causationSome of you may recall that iPhone 6 had a known issue with the CPU's failing.
Surprise surprise ... the T1 chip in 2016 / 2017 Touch Bar macs use ...? The A8 CPU used in the iPhone 6.
Obviously if a replacement Touch Bar or Display caused the issue it's unfair to blame it on the T1 chip.
On units in which it occurs spontaneously..? You do the math. (Still better than the nVidia debacles I guess).
'Machine 1 -- 15in 2017 MBPr 2.9GHz, 560, 1TB:'It had that stupid message pop up after doing a favor of replacing a fan. (How could swapping a damned fan have affected the T1 or Touch Bar!?). After HOURS of experimentation, I finally stumbled on to a process which after I figured out the exact sequence ... worked 100% of the time. I just needed 5 minutes with internet access to retry it ~4 (after every single reboot).
When the CRITICAL SOFTWARE UPDATE prompt came up:• Internet Connection Required (I have the LAN [RJ-45-to-TB2 ... and TB2-to-TB3 ] but ... WiFi works as well as wired)
• Attempts 2 or 3 NEVER work (no longer how long I'd wait before trying it)
• If I don't wait between attempts, 100 tries will not work.
• After 2-3 attempts,
wait ~3 minutes before trying & it'd (RELIABLY) install the update & load the OS.
• Do not exceed the 'time out' period; truly, 3-4 minutes I think is about the limit.
• After it loads the update the OS worked (literally) perfectly.
• Succeeding once changes nothing; you will need to install the update on every reboot (unless you choose safe mode).
An acquaintance who'd bought it from me asked I replace a noisy fan.
I'd received it 'ON' and shut it down (didn't test it's reboot behavior).
Perhaps the fan replacement was coincidental to the issue's emergence...
Perhaps my acquaintance withheld knowledge of the issue and exploited the fan swap to pretend it was new & my fault.
LUCKILY, I found a [functionally] "permanent fix" for the issue (recommended by my SMT Tech for Data Recovery.)
He said he found an article which said macOS 10.13 (HS) and 10.14 (Mojave) store the Touch Bar's update either on the T1 chip or the EFI maybe... but 10.15+ Catalina & Big Sur stored the update on a different component (the SSD?). As of 10.15+ it only stores the Fingerprint data on the T1 I think...which makes sense as the Touch Bar has no security vulnerability & isn't but a glorified USB 'mouse' if I understand correctly.
SHOCKINGLY ... his suggestion WORKED!The unit with the biggest PITA issue
was solved by upgrading the stupid OS to Big Sur.
'Machine 2'A 15in 2017 MBPr 3.1GHz, 560, 1TB
This unit's situation mild compared to the PITA-2.9GHz ... because it didn't 'hang' and wait for me to manually retry ~4 times; it'd just autonomously download the update (so long as it had WiFi info saved previously) it just made reboots a little slower.
Since the suggestion permanently resolved the 2.9GHz "PITA" I thought for sure updating the 3.1GHz to Big Sur would make it behave like a normally-functioning Mac ... right..? pff. yeah. right.
Upgraded to Big Sur & got the CRITICAL SOFTWARE UPDATE prompt (just as I'd had gotten once on the 2.9 also)...
But now, though it previously installed update on try-1 without requiring retries & waits... now? It wouldn't do it at all.
The 2.9GHz did require the update the first time Big Sur was installed, but it went off without a hitch ...
But now after repeated attempts & regardless of any repetition or waiting...nothing.
In fact, even rebooting to an external Mojave SSD (which is the OS it previously installed automatically) it still refuses.
Potential takeaway #1..? The suggestion (upgrading to Catalina or Big Sur fixes it by storing it differently) is just wrong.
Until I replicate the solution I'm going to categorize it as little more than a lucky coincidence.
But, as BF Skinner would've predicted; it worked before so I'm gonna try again!
(hopefully I don't become one of Skinner's maniacal pigeons relentlessly performing arbitrary behaviors (ie, looking over their left-shoulder while lifting their left leg) due to a
correlation-causation-fallacy of a random food distribution.)
Remaining attempts (that only cost time) I plan to try today:1. Boot from an external Big Sur image: a. Attempt booting from a Big Sur image which has already booted a Touch Bar MBPr
b. If step 'a' is successful I'll re-attempt booting from the upgraded SSD (maybe it'll load the driver to the EFI..?).
c. If step 'b' fails, I'll attempt target-booting the 3.1GHz to use as an external SSD to boot a Mid-2015 MBPr.
d. After c. I'll rinse and repeat some earlier attempts.
If all the above fails..?
a. Target boot (holding the 'T' key at 'power-on'.
b. Clone a sparse image of 10.14.6 via CCC to the local HD
c. Re-attempt the update locally with an OS I know previously worked.
*to my knowledge, SADLY! Mojave's the last OS you can clone via CCC without using Apple's installer! :'(Time permitting, I'll report back any successes & it should be assumed the above will fail as they're all tenuous solutionsStill, I strongly recommend (at least) trying: The repetition and timing to get the update to work as I did on System 1, the 2.9GHz.
If '1' fails, certainly attempt upgrading the OS to 10.15 or 11.2
If '2' fails & you've replaced an LCD or Touch Bar: Briefly return to OEM to try bypassing the issue & loading Big Sur.
Hope something helps...