Unifi USG L2TP VPN with DUO 2FA setup - Need help

I am struggling on setting up DUO 2FA on the L2TP VPN with DUO proxy on Windows server. The settings between Unifi and Windows DUO proxy is pretty straight forward. I have experience in setting DUO using Cisco ASA <> Windows but I was unable to make it happen on Unifi. Anyone has experience on this? I’ve seen this post: https://community.ui.com/questions/Duo-Security-Proxy-for-Windows-works-on-Unifi-a-few-notes/ceb6c9bb-73f4-4a38-83f3-c604323bfdbd

and somehow I got error every time when I connecting to VPN.

The error in the DUO authproxy.log:

Any help would be appreciated. Thanks