Your Windows 10 device may repeatedly attempt to reinstall the same update, which fails to install with error 0x800F801F.
This error may occur if a necessary file has been deleted. While enumerating missing components, the servicing stack checks the registry but not for the existence of certain files. If the file has been deleted, the servicing stack does not download the missing component, and the installation fails with CBS_E_SOURCE_MISSING. This does not trigger an automatic attempt to repair the missing files.
To resolve this issue, manually install the Windows Update from the Microsoft Update Catalog.
To do this, search for the update’s KB number at https://www.catalog.update.microsoft.com/home.aspx. Then download and install the appropriate update that applies to your device.