Hi,
I will walk through a few steps with you. Some of these are just to help figure out what is wrong, not to fix it right away. So if nothing changes after a step, that is normal. I will keep going until we solve it.
Download the offline package from https://catalog.s.download.windowsupdate.com/d/msdownload/update/software/secu/2026/09/windows10.0-kb5122878-x64_191c536a44656eb754daa7cb7e9b517b05d690f7.msu and try installing it. Let me know if it installs.
I look forward to hearing from you to get the results of the troubleshooting.