If you used Group Policy, disable the Group Policy setting that you used to enable Windows Defender Credential Guard (Computer Configuration -> Administrative Templates -> System -> Device Guard -> Turn on Virtualization Based Security).
Delete the following registry settings:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\LSA\LsaCfgFlags
HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\DeviceGuard\LsaCfgFlags
If you also wish to disable virtualization-based security delete the following registry settings:
HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\DeviceGuard\EnableVirtualizationBasedSecurity
HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\DeviceGuard\RequirePlatformSecurityFeatures
If you manually remove these registry settings, make sure to delete them all. If you don't remove them all, the device might go into BitLocker recovery.
Delete the Windows Defender Credential Guard EFI variables by using bcdedit. From an elevated command prompt, type the following commands:
mountvol X: /s
copy %WINDIR%\System32\SecConfig.efi X:\EFI\Microsoft\Boot\SecConfig.efi /Y
bcdedit /create {0cb3b571-2f2e-4343-a879-d86a476d7215} /d "DebugTool" /application osloader
bcdedit /set {0cb3b571-2f2e-4343-a879-d86a476d7215} path "\EFI\Microsoft\Boot\SecConfig.efi"
bcdedit /set {bootmgr} bootsequence {0cb3b571-2f2e-4343-a879-d86a476d7215}
bcdedit /set {0cb3b571-2f2e-4343-a879-d86a476d7215} loadoptions DISABLE-LSA-ISO
bcdedit /set {0cb3b571-2f2e-4343-a879-d86a476d7215} device partition=X:
mountvol X: /d