Error 789 RRAS L2TP/IPsec VPN server problem

Hi,

I am trying to build a basic L2TP VPN server in GCP cloud, first using WIn 2016 and then 2012R2 but same error in the end:

I have followed same trusted steps that always worked for me with on-prem environment but wont work this time.

I have done following troubleshoot but no Sol so far:

- Pre-shared is correct at both client machine and server, same key is working fine on existing VPN server.

- Restarted  IKE and AuthIPsec Keying Moudules and IPSec Policy Agent Services  - but no change

- made sure UDP port 500 and 4500 are open

- found nothing significant in Even viewers

please suggest.