WinSCP 6.1.1 error connecting to remote server

Version – 6.1.1
Windows Server 2022 64-bit
SFTP
GUI so far, automation to follow once it connects successfully
Error Message-

No supported authentication methods available (server sent: publickey)
Authentication log (see session log to details):
Using username "redacted"
Server refused our key
Authentication failed

Trying to connect to a partner remote server that uses RSA encryption. Generated private/public key pair on their interface and imported them into our environment. Loaded the private key to site manager and WinSCP converted it to PuTTY. I can connect to their SFTP via command line and the RSA key, but that won't work for our automation steps. Any ideas what we can do to resolve this? Partner says it's a problem on our side.

What do you mean by "I can connect to their SFTP via command line"? Do you mean OpenSSH sftp client? From the same local machine? Using the same key (the one you have converted to ppk)? Post its log file too.