It is an error of your operating system build rings not responding to action due to a bug. Microsoft faults it on the way it reads 'virtualized registry data' that allows mostly 32-bit systems to be bypassed. This is an issue with Windows update failing to find upgrade for Fast or slow ring when there is a build update. It is partly related to build ring failure error.
Image showing non-responding Fast-ring in grey
You may encounter this problem when the error prevents your computer from upgrading to the latest Windows 10 build and can sometimes cause the Fast or slow ring to grey out. Afterward, it displays the upgrade error code at the Windows update section for possible troubleshooting, right above the 'Check for updates' button when checking for Windows updates.
It is rare occurrence and when you experience it you'll have to take to rudimentary process of creating registry flag to forestall the problem.
When you are ready this guide will be helpful to you in finding a quick solution.
Note: It affects both 'fast and slow rings' when update is available to eligible users known as Windows insiders.
Image showing Start Menu and Windows Powershell(Admin) in Windows 10
STEP 1: Right-click on 'Start Menu' and select 'Command prompt (Admin)' or 'Windows Powershell (Admin)' - if you set it as your default shell.
Red arrow indicating the process
STEP 2: Copy and paste reg flags HKLM\Software\Microsoft\WindowsSelfHost\Applicability set DONT_VIRTUALIZE into command prompt or Windows Powershell and hit the ENTER key.
Image showing Windows 10 restarting
STEP 3: Restart your system for changes to take effect.
Red arrows pointing to Notifications icon and All settings
STEP 4: At the system tray click on 'Notifications' icon and select 'All settings' at the Windows flyout.
Image of red arrow indicating where to find 'Update & Security'
STEP 5: Click on 'Update & Security -Windows Update, recovery and backup' item to continue.
Image showing Check for updates in Windows 10
STEP 6: Next, click on 'Check for Updates' button to continue.