You would actually need to look at the logs from your FTP server, as it seems that there’s a permission error at that end. Kopia is only reporting its inability to write to that SFTP target.
Alright. On the provider I am using they don’t allow root access but via an account I am given.
Although, on the machine I wish to backup from is using the root user to perform the backup while the machine I wish to back to is using a specific non root user. Could that effect it?
Forget kopia for a moment and try with whatever ftp client you prefer (e.g. FileZilla). Does it work? Can you create dirs and send files? Using the same credentials you tried with kopia. If does not work then you have to solve your sftp access first.
So. I have tried via Username and Password and I was able to create and upload files using my storage provider account. Let me give public key authentication in Filezilla a try.