Macbook stuck in grey screen when using etcher USB

I am trying to install Proxmox on my old Macbook pro. When I use Rufus or imageUSB to create the install USB it boots into Grub rescue. But when I create the USB using Etcher the macbook is stuck on a grey screen while booting, and when I boot into the boot menu and then plugin the USB the Macbook freezes. Any idea what I should do?