seems the filesystem of the installer is corrupted.
my first guesses would be that either the ISO you downloaded got corrupted (verify the sha256 sum with the ones published at http://download.proxmox.com/iso/SHA256SUMS or that the USB drive has a problem (in that case try a different one)
idk who else would need my comment but I just bought a old dell poweredge r610 server and got this exact same error, turn out the bios version was too old. I updated to v6.6.0 and it worked