Windows 11 update keeps failing at the same percentage, then rolls back

A fixed percentage on rollback is the machine telling you which stage it chokes on, so it is repeatable and therefore findable. Look at the setup logs under C:$WINDOWS.~BT\Sources\Panther (setuperr.log and setupact.log) and search for the last driver or operation mentioned before the rollback. It is dull reading, but it usually names the culprit.

Easy fix. Delete everything in C:\Windows\SoftwareDistribution right away while the update is stuck, then restart and run it again. Also turn off Windows Update in services for good, so it stops trying and failing. Problem solved, no more error.

@Tyler B. the first part is close to what Microsoft's own steps say, but order matters: stop the Windows Update service first, then clear the folder, then start it again. And disabling Windows Update permanently is a bad idea, you stop getting security fixes. Fix the cause instead.

@Dmitri that was the way in. setuperr.log points at a driver for a network adapter that is several versions behind, which fits the C1900101 error. I should have gone to the logs before the third retry.

Before you go into logs, unplug everything: dock, external drives, USB hubs, a second monitor. We had a work laptop that failed the same way every month until the dock was disconnected, then it installed on the first try. It is a two minute test and it has fixed more updates for us than anything fancy.

C1900101 is almost always a driver, and on older machines the usual suspects are the display driver, the Wi-Fi or Bluetooth driver, and storage controller drivers. Get the current ones from the laptop maker's support page, not from a driver tool. Also check the disk. A failing SSD can stall an update at the same spot when it hits a bad block, so run chkdsk /f on C: and look at the drive's health status.

@Grace O. I did that too, no dock was attached but I had a USB Wi-Fi adapter plugged in from a test. I removed it, uninstalled the old network adapter driver in Device Manager, restarted, and the update finally completed on the next attempt. So it was the driver, not the disk.