I too have the same problem. I have installed the "2021-09 Cumulative Update for Windows 10 Version 21H1 for x64-based Systems (KB5005565)" update nearly 15 times.
Every time it completes 100% and after some time its shows error message stating that the update could not be installed.
After I restart my laptop it again downloading the update and does the same repeatedly.
Microsoft needs to fix it or remove this update from the server. This is becoming a daily nuisance and irritating.