I had this issue with a recent install and figured out what was wrong.
When issuing the dd command to move the iso to the thumb drive, I had specified "of=/dev/sdb1" instead of "of=/dev/sdb", so the ISO went onto partition 1 of the thumb drive instead of copying to the thumb drive complete with its own partition table.
The iso, at least as of version 9, uses 4 partitions - so making this mistake with the dd step meant it wasn't able to set up the partitions correctly.
To fix it:
When you're creating the bootable thumb drive, use a command like:
`dd bs=1M conv=fdatasync if=./proxmox-ve_*.iso of=/dev/sdx`
where you use the path to the iso you're using and changing x to specify whatever device you're intending to install on, WITHOUT any trailing number for any partitions that might be on the drive.
Using some of the other tools for creating bootable media might help avoid this kind of single-character error when using `dd`.
