Issue SFTP is not available for System User for Subscription

[pschlenker]

I have an Ubuntu VPS server running on an Amazon Lightsail instance with Plesk Obsidian installed. I can connect to the server via SSH and FTP using the user account specified by Amazon Lightsail, but I cannot SSH or FTP into the server using the system account of the subscription. I can SSH into the server with the user account specified by Amazon Lightsail, and then su to the system user account, but I can't log in initially using the system user account. When I try to FTP into the server using my FileZilla FTP client configured to log in using the system user account, I get as permission denied error. I have FileZilla configured to SFTP into the server using a key file.