This error first came up early in 2018, but I’ve stumbled across it a couple of times recently and had to hunt around for the solution, so I am posting it here so I know where to find it next time.
Microsoft released an update to address vulnerabilities for the Credential Security Support Provider protocol (CredSSP) used by Remote Desktop Protocol (RDP) connections for Windows clients and Windows Server.
Since that hotfix came out, if you try to connect from a patched machine to an unpatched one, you receive this error:
Remote Desktop Protocol
An authentication error has occurred.
The function requested is not supported.
Remote computer: {server name}
This could be due to CredSSP encryption oracle remediation.