How to fix "IKE authentication credentials are unacceptable" when connect to our VPN service?

You may get the below error when connecting to the our VPN service. This article will provide a solution for this issue. 
That is due to the client device did not have the below Let's encrypt trust root certificate installed. 
You can download the certificate here and double-click to install it on your local machine.
Then try to connect VPN again, it will work.