IKEv2 VPN not working after updating to MacOS Catalina - User authentication failed with username authentication settings
I had the same problem and I fixed with changing "Authentication Settings" from "Certificate" to "None". 
After this change, you will see 2 new options appeared just below, "Shared Secret" and "Certificate". Select "Certificate" option and reselect the certificate that is already used for the VPN connection before. 
Then click Connect button, confirm applying changes and viola!