From what I can tell, the Netbox plugin in XO syncs VMs, interfaces and IPs, but it doesn't populate Netbox's separate Virtual Disks model yet, which is why you get the total disk space but an empty Virtual Disks tab.
I don't think it's a permissions thing on your side, it looks more like a gap in what the plugin syncs today.
There's already an open issue for disk-size sync (https://github.com/vatesfr/xen-orchestra/issues/7643), plus a few other Netbox requests in the tracker. If you want the per-disk sync added, the place it actually gets weighed is https://feedback.vates.tech, where the Netbox asks get prioritised together.
It might also be worth a mention to @Team-XO-Backend, since they own the plugin and can say whether the newer Netbox virtual-disk model is on their radar.
ecoutinho created this issue in vatesfr/xen-orchestra
open
Netbox plugin: Sync disk size error
#7643