SFTP: Failed to create the sftp client

I’m using Duplicacy WEB as a docker container in Unraid

I’m trying to set up a SFTP backup storage against a QNAP Server:

But when trying to list the Directory, I get this error:

Failed to create the sftp client: ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey], no supported methods remain

What am I doing wrong?

I have also installed Duplicacy CLI on the Unraid Host OS, and there it works with no problems, the key works at least.

What could be wrong?