Upgrading win10 to win11: error during APPLY_IMAGE operation during SAFE_OS phase of installation

[MOoD90s 0]

I'm running a windows 10 system. My sole usage of the system is to run Intuit TurboTax. The system was originally a windows 7 system; Microsoft eventually required that I update to windows 10, which I did in January 2020.

Intuit announced that for TurboTax 2025, they required windows 11, and I am now attempting an upgrade. I ran Windows Update > Check for updates, and installed all of the remaining windows 10 updates available, then rebooted to make sure that everything was running ok. I also ran PC Health Check > Check now, which reported "This PC meets Windows 11 requirements"

I downloaded Win11_25H2_English_x64.iso, mounted it, and ran setup.exe. I attached an extra USB drive for use by the update (empty, 100GB available). The setup started and started counting percent complete, at about 38% per hour. Sometime around 2.6 hours, it rebooted, then displayed a screen (with a rotating "busy" icon) reading:

Working on updates 0%
Please keep your computer on

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I left it running for 10 hours, at which time it had progressed to ... 0%.

I found advice on the web suggesting a reboot. I did that, and the installation reported

Undoing changes made to your computer

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and it had indeed returned to windows 10.

A dialog reported:

We couldn't install Windows 11
We've set your PC back to the way it was right before you started installing Windows 11.

0xC1900101 - 0x2000C
The installation failed in the SAFE_OS phase with an error during APPLY_IMAGE operation

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I ran Settings > Update & Security > Troubleshoot > Additional troubleshooters, then under "Get up and running", selected Windows Update > Run the troubleshooter. It reported

Troubleshooting has completed
The troubleshooter has made some changes to your system. Try attempting the task you were trying to do before.

Click to expand...

I'm going to try it again, but I'm pessimistic.

My question: what does the dialog reporting the error in the APPLY_IMAGE operation of SAFE_OS mean? What needs fixing?