0xC0210021: STATUS_FVE_REBOOT_REQUIRED

Windows Error Code Lookup: Decode Windows error codes: Win32, HRESULT (0x80070005), NTSTATUS, Windows Update, msiexec and blue-screen stop codes, with what to check next.

Paste the number from an error dialog, event log, script or setup log: 0x80070005, 80070005, -2147024891, 1603, 0xC0000005, a stop code like 0x7E, a name like E_ACCESSDENIED, or a few words. You get every way Windows could mean that number (an HRESULT often wraps a Win32 error, so 0x80070005 is "Access is denied" too), its parts decoded, and what to check for the common ones. Names and messages come from the Windows SDK headers and Windows' own message tables; see Microsoft's System Error Codes and [MS-ERREF] for the full references.