If your VPS server has a Linux distribution (Centos, Ubuntu, Debian etc ..) your server username will be "root".
Remember that the username is case sensitive.
if you have a Windows server, the system user will be "Administrator". Check the username carefully in your RDP connection program
For connecting via SSH to your Linux VPS server, we recommend the Putty program (https://www.putty.org/)
For the Remote connection (RDP) for your Windows VPS server, the Windows operating system has a basic system application called "Remote Desktop Connection"