SFTP to remote server

Hi, I’d like to sync a folder from my nextcloud instance through SFTP with a remote server. I created a private key on my Linux client using ssh-keygen -t rsa and uploaded it to my instance.

This is how I configured my external storage:

As you see, I keep getting an error message “unable to load private key” despite having the PK stored in the .ssh folder (which is probably one of the unsafest things to do anyways).

What am I doing wrong and how do I configure SFTP correctly to have my folder (the one on the left) synced with the remote folder (/var/…)?

Thanks in advance!

Nextcloud is the client. Have it generate the client: