In my case it was a registry key that disallowed to install/update the beta of enpass:
You can check:
HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\PreviewBuilds\AllowBuildPreview
If set to 0, package flight updates are not available for you. After setting this to 1 the beta was found and installed.
Same here - joined yesterday and received "successfully registered for Enpass beta program" for "Windows 10-Desktop" and my Microsoft ID. No Beta 6 in the App Store.