This bug was fixed on Patch Tuesday.
Mentioned just on the offhand chance this ever happens in the future...
There was a bug [yeah, one of many] in earlier versions of Win10, where some files in the Windows folder might use filenames that Windows wouldn't work with. After upgrading Win10 versions requiring a reinstall, those files were moved to Windows.old, and because of them Windows.old could not be deleted. They had to be deleted booting to a USB stick using the Command Prompt. It was a real joy on my tablets since not using Windows meant the display wasn't enlarged 150% -- needed a magnifying glass!
Otherwise, there's something I always add to Windows... added to the context menu, it lets you take ownership of files/folders, including nested, so you have permissions to delete them. Create a simple .reg file in Notepad, then merge [double click] with the registry.
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Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\*\shell\runas]
@="Take Ownership"
"NoWorkingDirectory"=""
[HKEY_CLASSES_ROOT\*\shell\runas\command]
@="cmd.exe /c takeown /f \"%1\" && icacls \"%1\" /grant administrators:F"
"IsolatedCommand"="cmd.exe /c takeown /f \"%1\" && icacls \"%1\" /grant administrators:F"
[HKEY_CLASSES_ROOT\Directory\shell\runas]
@="Take Ownership"
"NoWorkingDirectory"=""
[HKEY_CLASSES_ROOT\Directory\shell\runas\command]
@="cmd.exe /c takeown /f \"%1\" /r /d y && icacls \"%1\" /grant administrators:F /t"
"IsolatedCommand"="cmd.exe /c takeown /f \"%1\" /r /d y && icacls \"%1\" /grant administrators:F /t"
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