IKEv2 VPN on macOs 10.12 Sierra
In my situation, setting server-side in ipsec.conf:

 leftsendcert=always


solved the problem.

From ipsec.conf documentation:


  left|rightsendcert = never | no | ifasked | always | yes
  
  Accepted values are never or no, always or yes, and ifasked, the
  latter meaning that  the peer must send a certificate request (CR)
  payload in order to get a certificate in return.


Moreover, I had to add to the Keychain the root certificate and trust it in order to validate the certificate sent by the server and I read that if you have intermediate certificates you have to trust them too.