To find what is causing the upgrade failure, you have to look at the logs. Different logs are generated at each phase of the upgrade.
Posssibly, something will jump out at the time of the rollback. They are in Microsoft speak so I can't say if everything can be interpreted.
When I was upgrading 23h2 to 24h2, Mine kept rolling back so I found my particular error in the last 2 logs. It indicated there was some issue with my user account but I could not decipher what the issue was so I ended up clean installing. Hopefully you will have better luck. These instructions came from MS.
- WIINDOWS UPGRADE FAILURES
If no error message occurs, the following log files are created when an upgrade fails during installation before the computer restarts for the second time. You have to look at these logs to get an idea of what's causing the failure.
C:\$Windows.~BT\Sources\panther\setupact.log
C:\$Windows.~BT\Sources\panther\miglog.xml
C:\Windows\setupapi.log
[Windows 10:] C:\Windows\Logs\MoSetup\BlueBox.log
C:\Windows\inf\setupapi.app.log
C:\Windows\inf\setupapi.dev.log
C:\Windows\panther\PreGatherPnPList.log
C:\Windows\panther\PostApplyPnPList.log
C:\Windows\memory.dmp
The following log files are created when an upgrade fails, and then you restore the desktop:
C:\$Windows.~BT\Sources\panther\setupact.log
C:\$Windows.~BT\Sources\panther\miglog.xml
C:\$Windows.~BT\sources\panther\setupapi\setupapi.dev.log
C:\$Windows.~BT\sources\panther\setupapi\setupapi.app.log
C:\Windows\memory.dmp
The following log files are created when an upgrade fails, and the installation rollback is initiated:
C:\$Windows.~BT\Sources\Rollback\setupact.log
C:\$Windows.~BT\Sources\Rollback\setupact.err
If logfiles do not direct you to what caused the error, I advise anyone to not waste his time. Either stay where you are or bite the bullet. back up your data, and do a clean install. Upgrade failures will have you chasing your tail for days.