Proxmox Default Username is root.
Proxmox VE uses root as the default administrative username. During installation, you create a password for this account, which is then used to sign in to the Proxmox web interface and manage the server. The default authentication realm is typically PAM, so the full login identity is commonly written as root@pam. Proxmox does not provide a universal default password; you set the root password during installation.
When accessing Proxmox through the web interface, enter root as the username and select Linux PAM standard authentication (or PAM) as the realm. For security, avoid exposing the root account unnecessarily and use strong authentication credentials. Proxmox also supports creating additional users and assigning appropriate permissions, which can be preferable for routine administration rather than sharing the root account.
Did you forget Proxmox Default Port ?