While attempting to update my M2 MacBook Pro from macOS Tahoe v26.0.1 to v26.1, the update failed and triggered a continuous boot loop. The Mac would power on, show the Apple logo without a progress bar, turn off, turn on again, and repeat this cycle until eventually displaying an exclamation mark inside a circle.
According to Apple’s support page, this error requires placing the affected Mac into DFU mode and attempting a revive using another Mac.
Although I followed the key-combination instructions from Apple’s documentation, I noticed something was wrong. Even while holding the required keys, the Mac continued its boot loop and returned to the same error screen.
After connecting my Mac to another Mac and following several boot loops, when my Mac reached the above screen, it appeared on the other Mac, as seen in Apple's documentation here and I was presented with the "Revive" and "Restore" options. I tried both, unsuccessfully, each time leading to the error "The Mac "Мас" could not be updated. An unknown error occurred (21)."
Searching online it might be that error 21 is a generic error that can be displayed for various faults.
SOLUTION
In my case, the issue was that, even though the Mac appeared to connect to the other Mac, it was not actually entering DFU mode. I confirmed this using Apple's own Apple Configurator app (search in the App Store). If the connected Mac appears with a normal device icon, it is not in DFU mode. What you want to see is the DFU icon:
To reliably enter DFU mode, without finger gymnastics and counting seconds, use the macvdmtool by following the instructions. Once the Mac enters DFU mode successfully, you should be seeing a blank screen on the target Mac and the DFU icon in the other Mac within Apple Configurator. Right click the DFU icon and initiate a "Revive" or "Restore" as appropriate.
