How to Fix Proxmox (PVE) 6.4-15 to 7.x Upgrade Stuck at 57%

Just yesterday, I received a task to install a software on our client's private/local server at their office. They were using Proxmox to create Containers (Virtual Machines) on it. However, as they were using Proxmox version 6.4-15 at that time and i...