Maybe case-sensitive problem.
Try change "image": "/ISO/ubuntu-21.04-desktop-amd64.iso" to "image": "/iso/ubuntu-21.04-desktop-amd64.iso",
Recommend to use VentoyPlugson to automatically generate ventoy.json to avoid problem.
https://www.ventoy.net/en/plugin_plugson.html
Try change "image": "/ISO/ubuntu-21.04-desktop-amd64.iso" to "image": "/iso/ubuntu-21.04-desktop-amd64.iso",
Recommend to use VentoyPlugson to automatically generate ventoy.json to avoid problem.
https://www.ventoy.net/en/plugin_plugson.html